[flang] To use LLVM's libc++ instead of GCC's libstdc++, define

BUILD_WITH_CLANG_LIBRARIES on the cmake command line to
be the path to a clang installation with bin, lib, etc.
For this to work, BUILD_WITH_CLANG must be defined too.

Original-commit: flang-compiler/f18@a36d5fee421f2e90293540335c5b887412e31383
Reviewed-on: https://github.com/flang-compiler/f18/pull/326