[CMake] Fix issues building runtimes

This resolves two issues:
(1) LIBCXX_HEADER_DIR is a very misleadingly named variable because it shouldn't be set to the header directory, instead it needs to be the root binary dir.
(2) If you build runtimes without libcxx, we can't depend on the libcxx header target, so we should instaed refer to it by the variable name which will be unset if libcxx isn't present.

llvm-svn: 361646