As discussed on python-dev, don't remove Python-ast.[ch] with distclean

even though they are generated.  Since these files require Python 2.2+,
it's possible they can't be created on a fresh system.

Makefile.pre.in

@@ -988,10 +988,10 @@ clobber: clean
 
 # Make things extra clean, before making a distribution:
 # remove all generated files, even Makefile[.pre]
+# Keep configure and Python-ast.[ch], it's possible they can't be generated
 distclean: clobber
 	-rm -f core Makefile Makefile.pre buildno config.status \
 		Modules/Setup Modules/Setup.local Modules/Setup.config
-	-rm -f $(AST_H) $(AST_C)
 	find $(srcdir) '(' -name '*.fdc' -o -name '*~' \
 			   -o -name '[@,#]*' -o -name '*.old' \
 			   -o -name '*.orig' -o -name '*.rej' \