Everything was OK as long as GMutex was backed by pthread_mutex_t on
POSIX.  Since this is no longer the case, the ABI of GStaticMutex was
broken.

Fix that up by using pthread_mutex_t directly in the structure.  Since
that's potentially incompatible with our GMutex implementation, set
g_thread_use_default_impl to FALSE to cause the fallback code (which
manually allocates a GMutex) to run, even in the case of
already-existing code (without the need for a recompile).  This will
cause the pthread_mutex_t part of the struct to be completely ignored.