Use pthreads on Windows as well

We now use a version of MinGW that has libpthread.a and all its friends.
Start compiling using that in anticipation of a new roll of BoringSSL
that starts using pthreads on Windows.

Test: mmma -j32 external/boringssl/
Change-Id: Ia74b5f61b3d028f20827d5fb22804c598a54ee97
1 file changed