check for this pulls in cmake modules which check for LLVM toolchain
components (llvm-ar/llvm-ranlib) which are only present in the version
of LLVM in ports not the version in base; it then tries to use them in
the build, e.g. llvm-ar instead of ar when archiving libraries.
There is a cmake variable _CMAKE_TOOLCHAIN_LOCATION which looks like
it should force only checking in a certain path, but it doesn't seem
to work.
Build problem reported by tb@
59f1a1315c