[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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291433 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed