diff --git a/Makefile b/Makefile
index 12c403fb..67ba9d25 100644
--- a/Makefile
+++ b/Makefile
@@ -73,7 +73,7 @@ flags/libs.tmp: flags/libs.tmp.in
 	opam config subst $@
 
 flags/libs: flags/libs.tmp Makeconf
-	env PKG_CONFIG_PATH="$(shell opam config var prefix)/lib/pkgconfig" \
+	env PKG_CONFIG_PATH="$(PREFIX)/lib/pkgconfig" \
 	    pkg-config $(PKG_CONFIG_DEPS) --libs >> $<
 	awk -v RS= -- '{ \
 	    sub("@@PKG_CONFIG_EXTRA_LIBS@@", "$(PKG_CONFIG_EXTRA_LIBS)", $$0); \
@@ -84,7 +84,7 @@ flags/cflags.tmp: flags/cflags.tmp.in
 	opam config subst $@
 
 flags/cflags: flags/cflags.tmp Makeconf
-	env PKG_CONFIG_PATH="$(shell opam config var prefix)/lib/pkgconfig" \
+	env PKG_CONFIG_PATH="$(PREFIX)/lib/pkgconfig" \
 	    pkg-config $(PKG_CONFIG_DEPS) --cflags >> $<
 	awk -v RS= -- '{ \
 	    print "(", $$0, ")" \
diff --git a/configure.sh b/configure.sh
index f34a601f..9b577a06 100755
--- a/configure.sh
+++ b/configure.sh
@@ -1,6 +1,11 @@
 #!/bin/sh
 
-export PKG_CONFIG_PATH=$(opam config var prefix)/lib/pkgconfig
+PREFIX=${1:-$PREFIX}
+if [ "$PREFIX" = "" ]; then
+  PREFIX="$(opam config var prefix)"
+fi
+
+export PKG_CONFIG_PATH="${PREFIX}/lib/pkgconfig"
 pkg_exists() {
     pkg-config --exists "$@"
 }
diff --git a/opam b/opam
index 95b424a2..b23e7a99 100644
--- a/opam
+++ b/opam
@@ -6,15 +6,14 @@ bug-reports: "https://github.com/mirage/ocaml-freestanding/issues/"
 license: "MIT"
 tags: "org:mirage"
 dev-repo: "git+https://github.com/mirage/ocaml-freestanding.git"
-build: [make]
+build: [make "PREFIX=%{prefix}%"]
 install: [make "install" "PREFIX=%{prefix}%"]
-remove: [make "uninstall" "PREFIX=%{prefix}%"]
 depends: [
   "conf-pkg-config"
   "ocamlfind" {build}
   "ocaml-src" {build}
   ("solo5-bindings-hvt" | "solo5-bindings-spt" | "solo5-bindings-virtio" | "solo5-bindings-muen" | "solo5-bindings-genode")
-  "ocaml" {>= "4.08.0" & < "4.11.0"}
+  "ocaml" {>= "4.08.0" & < "4.12.0"}
 ]
 substs: [
   "flags/cflags.tmp"