Make better use of GNU Pth -- patch by Andy Dustman.

I can't test this, so I'm just checking it in with blind faith in Andy.
I've tested that it doesn't broeak a non-Pth build on Linux.

Changes include:

- There's a --with-pth configure option.

- Instead of _GNU_PTH, we test for HAVE_PTH.

- Better signal handling.

- (The config.h.in file is regenerated in a slightly different order.)

Python/thread.c

@@ -109,13 +109,14 @@ void PyThread_init_thread(void)
 #include "thread_lwp.h"
 #endif
 
-#ifdef _GNU_PTH
+#ifdef HAVE_PTH
 #include "thread_pth.h"
-#else
+#undef _POSIX_THREADS
+#endif
+
 #ifdef _POSIX_THREADS
 #include "thread_pthread.h"
 #endif
-#endif
 
 #ifdef C_THREADS
 #include "thread_cthread.h"