[libcxx] Fix externally-threaded shared library builds after r291275.

Need to allow unresolved symbols in the dylib. This was previously done for
LIBCXX_HAS_EXTERNAL_THREAD_API, but we have since split that into two with
LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY being the externally-threaded variant.

Also a minor CMakeLists.txt cleanup.

llvm-svn: 291433