mk: tell rustc that we're only looking for native libs in the LLVM_LI…

…BDIR

This fixes the case where we try to re-build & re-install rust to the
same prefix (without uninstalling) while using an llvm-root that is the
same as the prefix.

Without this, builds like that fail with:
	'error: multiple dylib candidates for `std` found'

See https://github.com/jmesmon/meta-rust/issues/6 for some details.

May also be related to rust-lang#20342.

mk/main.mk

@@ -294,7 +294,7 @@ LLVM_VERSION_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --version)
 LLVM_BINDIR_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --bindir)
 LLVM_INCDIR_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --includedir)
 LLVM_LIBDIR_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --libdir)
-LLVM_LIBDIR_RUSTFLAGS_$(1)=-L "$$(LLVM_LIBDIR_$(1))"
+LLVM_LIBDIR_RUSTFLAGS_$(1)=-L native="$$(LLVM_LIBDIR_$(1))"
 LLVM_LDFLAGS_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --ldflags)
 ifeq ($$(findstring freebsd,$(1)),freebsd)
 # On FreeBSD, it may search wrong headers (that are for pre-installed LLVM),