[libcxx] Only declare contents of threading API when
_LIBCPP_HAS_THREAD_API_EXTERNAL is not defined.

When it is defined they will be declared by the
__external_threading header instead.

Differential revision: https://reviews.llvm.org/D66518

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369537 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed