Add dependencies for Python/thread.c on all of the header files that

it may depend on.  It's really annoying that thread.o doesn't get
rebuilt when the .h file is changed! :-)

The dependency is on *all* the Python/thread_*.h files -- that should
be sufficient and rarely cause unneeded recompilations.

Makefile.pre.in

@@ -881,4 +881,8 @@ funny:
 		-o -name MANIFEST \
 		-o -print
 
+# Dependencies
+
+Python/thread.o: $(srcdir)/Python/thread_*.h
+
 # IF YOU PUT ANYTHING HERE IT WILL GO AWAY