diff --git a/.gitignore b/.gitignore
index 0302eec4d..6cd1c6d30 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,7 +1,9 @@
 *.o
 *.swp
 *.pcap
+Makeconf
 cscope.*
+include-host/
 iso/boot/kernel
 tests/*/*.ukvm
 tests/*/*.virtio
diff --git a/Makefile b/Makefile
index ae6b79650..162b8e0f5 100644
--- a/Makefile
+++ b/Makefile
@@ -15,24 +15,37 @@
 # TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 # PERFORMANCE OF THIS SOFTWARE.
 
+export TOP=$(abspath .)
+$(TOP)/Makeconf:
+	./configure.sh
+include Makefile.common
+
 .PHONY: all
 all: ukvm virtio
+.DEFAULT_GOAL := all
 
 .PHONY: virtio
 virtio:
+ifeq ($(BUILD_VIRTIO), yes)
 	$(MAKE) -C kernel virtio
 	$(MAKE) -C tests virtio
+endif
 
 .PHONY: ukvm
 ukvm:
+ifeq ($(BUILD_UKVM), yes)
 	$(MAKE) -C kernel ukvm
 	$(MAKE) -C tests ukvm
+endif
 
+.PHONY: clean
 clean:
 	$(MAKE) -C kernel clean
 	$(MAKE) -C tests clean
 	$(RM) solo5-kernel-virtio.pc
 	$(RM) solo5-kernel-ukvm.pc
+	$(RM) -r include-host/
+	$(RM) Makeconf
 
 PREFIX?=/nonexistent # Fail if not run from OPAM
 OPAM_BINDIR=$(PREFIX)/bin
@@ -43,17 +56,18 @@ OPAM_VIRTIO_INCDIR=$(PREFIX)/include/solo5-kernel-virtio/include
 
 # We want the MD CFLAGS and LDFLAGS in the .pc file, where they can be
 # picked up by the Mirage tool / other downstream consumers.
-KERNEL_MD_CFLAGS=$(shell $(MAKE) -sC kernel print-md-cflags)
-KERNEL_LDFLAGS=$(shell $(MAKE) -sC kernel print-ldflags)
 %.pc: %.pc.in
 	sed <$< > $@ \
-	    -e 's#!CFLAGS!#$(KERNEL_MD_CFLAGS)#g;' \
-	    -e 's#!LDFLAGS!#$(KERNEL_LDFLAGS)#g;'
+	    -e 's#!CFLAGS!#$(MD_CFLAGS)#g;' \
+	    -e 's#!LD!#$(LD)#g;' \
+	    -e 's#!LDFLAGS!#$(LDFLAGS)#g;'
 
 .PHONY: opam-virtio-install
 opam-virtio-install: solo5-kernel-virtio.pc virtio
 	mkdir -p $(OPAM_VIRTIO_INCDIR) $(OPAM_VIRTIO_LIBDIR)
 	cp kernel/solo5.h $(OPAM_VIRTIO_INCDIR)/solo5.h
+	mkdir -p $(OPAM_VIRTIO_INCDIR)/host
+	cp -R include-host/. $(OPAM_VIRTIO_INCDIR)/host
 	cp iso/boot/grub/menu.lst $(OPAM_VIRTIO_LIBDIR)
 	cp iso/boot/grub/stage2_eltorito $(OPAM_VIRTIO_LIBDIR)
 	cp kernel/virtio/solo5.o kernel/virtio/solo5.lds $(OPAM_VIRTIO_LIBDIR)
@@ -73,6 +87,8 @@ opam-ukvm-install: solo5-kernel-ukvm.pc ukvm
 	mkdir -p $(OPAM_UKVM_INCDIR) $(OPAM_UKVM_LIBDIR)
 	cp kernel/solo5.h $(OPAM_UKVM_INCDIR)/solo5.h
 	cp ukvm/ukvm.h $(OPAM_UKVM_INCDIR)/ukvm.h
+	mkdir -p $(OPAM_UKVM_INCDIR)/host
+	cp -R include-host/. $(OPAM_UKVM_INCDIR)/host
 	cp kernel/ukvm/solo5.o kernel/ukvm/solo5.lds $(OPAM_UKVM_LIBDIR)
 	mkdir -p $(OPAM_BINDIR)
 	mkdir -p $(OPAM_UKVM_LIBDIR)/src
diff --git a/Makefile.common b/Makefile.common
new file mode 100644
index 000000000..0369a22fc
--- /dev/null
+++ b/Makefile.common
@@ -0,0 +1,15 @@
+# This Makefile defines global defaults for building Solo5 (kernel/) and the
+# in-tree test programs. These can be overriden either on the command line, or
+# via Makeconf generated by configure.sh.
+include $(TOP)/Makeconf
+
+AS?=as
+CC?=cc
+LD?=ld
+OBJCOPY?=objcopy
+# Exported to OPAM via pkg-config.
+MD_CFLAGS=$(HOST_CFLAGS) -ffreestanding -mno-red-zone -mno-3dnow
+# Likewise.
+LDFLAGS=-nostdlib -z max-page-size=0x1000 -static
+# CFLAGS used for building kernel/ and in-tree tests.
+CFLAGS=$(MD_CFLAGS) -isystem $(TOP)/include-host -std=gnu99 -Wall -Wextra -Werror -O2 -g
diff --git a/configure.sh b/configure.sh
new file mode 100755
index 000000000..8bfaaab72
--- /dev/null
+++ b/configure.sh
@@ -0,0 +1,84 @@
+#!/bin/sh
+
+die()
+{
+    echo "$0: $@" 1>&2
+    exit 1
+}
+
+cc_is_clang()
+{
+    ${CC:-cc} -v 2>&1 | grep -q "clang version"
+}
+
+cc_is_gcc()
+{
+    ${CC:-cc} -v 2>&1 | grep -q "^gcc version"
+}
+
+check_ld_version()
+{
+    type $1 >/dev/null 2>&1 && $1 -v | grep -q 'GNU.*2.2[5-9]'
+}
+
+# Host-provided header files are installed here for in-tree builds. OPAM will
+# install these to $(OPAM_INCDIR)/host where they will be picked up by
+# pkg-config.
+HOST_INCDIR=${PWD}/include-host
+
+case $(uname -s) in
+    Linux)
+        # On Linux/gcc we use -nostdinc and copy all the gcc-provided headers.
+        cc_is_gcc || die "Only 'gcc' is supported on Linux"
+        CC_INCDIR=$(${CC:-cc} -print-file-name=include)
+        [ -d "${CC_INCDIR}" ] || die "Cannot determine gcc include directory"
+        mkdir -p ${HOST_INCDIR}
+        cp -R ${CC_INCDIR}/. ${HOST_INCDIR}
+
+        HOST_CFLAGS="-nostdinc"
+        BUILD_UKVM="yes"
+        BUILD_VIRTIO="yes"
+        ;;
+    FreeBSD)
+        # On FreeBSD/clang we use -nostdlibinc which gives us access to the
+        # clang-provided headers for compiler instrinsics. We copy the rest
+        # (std*.h, float.h and their dependencies) from the host.
+        cc_is_clang || die "Only 'clang' is supported on FreeBSD"
+        INCDIR=/usr/include
+        SRCS_MACH="machine/_stdint.h machine/_types.h machine/endian.h \
+            machine/_limits.h"
+        SRCS_SYS="sys/_null.h sys/_stdint.h sys/_types.h sys/cdefs.h \
+            sys/endian.h"
+        SRCS_X86="x86/float.h x86/_stdint.h x86/stdarg.h x86/endian.h \
+            x86/_types.h x86/_limits.h"
+        SRCS="float.h osreldate.h stddef.h stdint.h stdbool.h stdarg.h"
+
+        mkdir -p ${HOST_INCDIR}
+        mkdir -p ${HOST_INCDIR}/machine ${HOST_INCDIR}/sys ${HOST_INCDIR}/x86
+        for f in ${SRCS_MACH}; do cp -f ${INCDIR}/$f ${HOST_INCDIR}/machine; done
+        for f in ${SRCS_SYS}; do cp -f ${INCDIR}/$f ${HOST_INCDIR}/sys; done
+        for f in ${SRCS_X86}; do cp -f ${INCDIR}/$f ${HOST_INCDIR}/x86; done
+        for f in ${SRCS}; do cp -f ${INCDIR}/$f ${HOST_INCDIR}; done
+
+        HOST_CFLAGS="-nostdlibinc"
+        BUILD_UKVM=
+        BUILD_VIRTIO="yes"
+        # Check that the linker is new enough. Allow the user to explicitly
+        # override ${LD}; if unset default to looking for the binutils one.
+        LD=${LD:-/usr/local/bin/ld}
+        check_ld_version ${LD} || \
+            die "Linker '${LD}': Not found or too old (< 2.25)." \
+            "Please install ld from 'binutils' or set \$LD explicitly."
+        EXTRA="LD=${LD}"
+        ;;
+    *)
+        die "Unsupported build OS: $(uname -s)"
+        ;;
+esac
+
+cat <<EOM >Makeconf
+BUILD_UKVM=${BUILD_UKVM}
+BUILD_VIRTIO=${BUILD_VIRTIO}
+HOST_CFLAGS=${HOST_CFLAGS}
+${EXTRA}
+EOM
diff --git a/kernel/Makefile b/kernel/Makefile
index ecad781e3..a8a7e5dfd 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -15,7 +15,7 @@
 # TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 # PERFORMANCE OF THIS SOFTWARE.
 
-include Makefile.common
+include $(TOP)/Makefile.common
 
 COMMON_COBJS=\
 abort.o \
@@ -80,12 +80,3 @@ ukvm/solo5.o: $(UKVM_COBJS) ukvm/solo5.lds
 .PHONY: clean
 clean:
 	$(RM) *.o virtio/*.o ukvm/*.o kernel 
-
-# This target is for the top-level opam-install to get MD CFLAGS
-.PHONY: print-md-cflags
-print-md-cflags:
-	@echo $(MD_CFLAGS)
-
-.PHONY: print-ldflags
-print-ldflags:
-	@echo $(LDFLAGS)
diff --git a/kernel/Makefile.common b/kernel/Makefile.common
deleted file mode 100644
index 3b4ba2180..000000000
--- a/kernel/Makefile.common
+++ /dev/null
@@ -1,14 +0,0 @@
-AS?=as
-CC?=cc
-LD?=ld
-OBJCOPY?=objcopy
-# Ensure no host headers or libraries are used when building, with the
-# exception of:
-#   - Standard C headers required by C89/C99. -nostdinc removes access to these
-#     so get it back by asking GCC for its installation directory.
-GCC_INCLUDE_DIR=$(shell $(CC) -print-file-name=include)
-MD_CFLAGS=-nostdinc -isystem $(GCC_INCLUDE_DIR) \
-	  -ffreestanding -mno-red-zone -mno-3dnow
-LDFLAGS=-nostdlib -z max-page-size=0x1000 -static
-# Platform-independent CFLAGS
-CFLAGS=$(MD_CFLAGS) -std=gnu99 -Wall -Wextra -Werror -O2 -g
diff --git a/solo5-kernel-ukvm.pc.in b/solo5-kernel-ukvm.pc.in
index 422a90da1..3d6244a96 100644
--- a/solo5-kernel-ukvm.pc.in
+++ b/solo5-kernel-ukvm.pc.in
@@ -2,9 +2,10 @@ prefix=${pcfiledir}/../..
 exec_prefix=${prefix}
 includedir=${prefix}/include/solo5-kernel-ukvm/include
 libdir=${exec_prefix}/lib/solo5-kernel-ukvm
+ld=!LD!
 ldflags=!LDFLAGS! -T ${libdir}/solo5.lds ${libdir}/solo5.o
 
 Name: solo5-kernel-ukvm
 Version: 0.1
 Description: Solo5 unikernel base (ukvm target)
-Cflags: !CFLAGS! -I${includedir}
+Cflags: !CFLAGS! -isystem ${includedir}/host -I${includedir}
diff --git a/solo5-kernel-virtio.opam b/solo5-kernel-virtio.opam
index 1067d0a23..4dbccc3d4 100644
--- a/solo5-kernel-virtio.opam
+++ b/solo5-kernel-virtio.opam
@@ -18,6 +18,10 @@ conflicts: [
   "solo5-kernel-ukvm"
 ]
 
+depexts: [
+  [["freebsd"] ["binutils"]]
+]
+
 available: [
   ocaml-version >= "4.02.3" & (arch = "x86_64" | arch = "amd64") & os != "darwin"
 ]
diff --git a/solo5-kernel-virtio.pc.in b/solo5-kernel-virtio.pc.in
index acc8a6844..81bb54e0b 100644
--- a/solo5-kernel-virtio.pc.in
+++ b/solo5-kernel-virtio.pc.in
@@ -2,9 +2,10 @@ prefix=${pcfiledir}/../..
 exec_prefix=${prefix}
 includedir=${prefix}/include/solo5-kernel-virtio/include
 libdir=${exec_prefix}/lib/solo5-kernel-virtio
+ld=!LD!
 ldflags=!LDFLAGS! -T ${libdir}/solo5.lds ${libdir}/solo5.o
 
 Name: solo5-kernel-virtio
 Version: 0.1
 Description: Solo5 unikernel base (virtio target)
-Cflags: !CFLAGS! -I${includedir}
+Cflags: !CFLAGS! -isystem ${includedir}/host -I${includedir}
diff --git a/tests/Makefile.tests b/tests/Makefile.tests
index 18913e7b5..22a742876 100644
--- a/tests/Makefile.tests
+++ b/tests/Makefile.tests
@@ -1,22 +1,23 @@
+include $(TOP)/Makefile.common
+
 .PHONY: ukvm
 ukvm: $(UKVM_TARGETS)
 
 .PHONY: virtio
 virtio:	$(VIRTIO_TARGETS)
 
-SOLO5_DIR=../../kernel
-UKVM_SRC=../../ukvm
+SOLO5_DIR=$(TOP)/kernel
+UKVM_SRC=$(TOP)/ukvm
 
-include $(SOLO5_DIR)/Makefile.common
 CFLAGS+=-I$(SOLO5_DIR)
 
 %.o: %.c $(HEADERS)
 	$(CC) $(CFLAGS) -c $< -o $@
 
+ifeq ($(BUILD_UKVM), yes)
 Makefile.ukvm: $(UKVM_SRC)/ukvm-configure
 	$(UKVM_SRC)/ukvm-configure $(UKVM_SRC) $(UKVM_MODULES)
 
-.PHONY: ukvm-clean
 -include Makefile.ukvm
 
 %.ukvm: %.o $(SOLO5_DIR)/ukvm/solo5.lds $(SOLO5_DIR)/ukvm/solo5.o
@@ -25,6 +26,7 @@ Makefile.ukvm: $(UKVM_SRC)/ukvm-configure
 
 $(SOLO5_DIR)/ukvm/solo5.o: 
 	$(MAKE) -C $(SOLO5_DIR) ukvm
+endif
 
 %.virtio: %.o $(SOLO5_DIR)/virtio/solo5.lds $(SOLO5_DIR)/virtio/solo5.o
 	$(LD) -T $(SOLO5_DIR)/virtio/solo5.lds \
@@ -33,6 +35,6 @@ $(SOLO5_DIR)/ukvm/solo5.o:
 $(SOLO5_DIR)/virtio/solo5.o: 
 	$(MAKE) -C $(SOLO5_DIR) virtio
 
-.PHONY: clean
+.PHONY: clean ukvm-clean
 clean: ukvm-clean
 	$(RM) *.o $(UKVM_TARGETS) Makefile.ukvm $(VIRTIO_TARGETS)