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.)

Include/Python.h

@@ -111,7 +111,7 @@
 #define Py_file_input 257
 #define Py_eval_input 258
 
-#ifdef _GNU_PTH
+#ifdef HAVE_PTH
 /* GNU pth user-space thread support */
 #include <pth.h>
 #endif