This enables the use of modules for a significant build performance gain
when building with clang as host compiler or when using HAVE_LLVM=yes.
Switch libc++ to using the copy from the mono-repo.
programs there; make all Makefiles that use bsd.hostprog.mk include it.
Namely turn off MKREPRO and don't make lint, man pages, info files etc.
Remove the Makefile.inc files that contained these same settings, and
remove the settings from Makefile.host
Upstream sources can be fetched by running "make checkout" in
src/external/bsd/llvm, they will be properly imported once the
integration and missing features are sorted out.