thread: Support pthreads-win32. * lib/glthread/thread.h (gl_thread_self): Define differently on pthreads-win32. (gl_null_thread): New declaration. (gl_thread_self_pointer): New macro. * lib/glthread/thread.c (gl_null_thread): New constant. * tests/test-lock.c: Use gl_thread_self_pointer instead of gl_thread_self. * tests/test-tls.c: Likewise. Suggested by Paul Eggert. Reported by Eric Blake.