configure.ac defined G_THREAD_SOURCE and gthread-impl would #include it.

Instead, since we only have two thread implementations now, and since we
always use the Windows one only on Windows, move the logic to the
Makefile, predicated on 'if OS_WIN32'.  Then have the chosen backend do
the #include "gthread-impl.c" from there.

Remove the G_THREAD_SOURCE define from configure.ac.