[libcxx] Fix testing of the externally-threaded library build

 after r290850

Before r290850, building libcxx with -DLIBCXX_HAS_EXTERNAL_THREAD_API=ON had two
uses:
  - Allow platform vendors to plug-in an __external_threading header which
    should take care of the entire threading infrastructure of libcxx

  - Allow testing of an externally-threaded library build; where the thread API
    is declared using pthread data structures, and the implementation of this
    API is provided as a separate library (test/support/external_threads.cpp)
   and linked-in when running the test suite.

r290850 breaks the second use case (pthread data structures are no longer
available). This patch re-stores the ability to build+test an
externally-threaded library variant on a pthread based system.

llvm-svn: 290878