SF #1373150, diffs in working copy after a build

Strip off leading dots and slash so the generated files are the same regardless
of whether you configure in the checkout directory or build.

If anyone configures in a different directory, we might want a cleaner
approach using os.path.*().  Hopefully this is good enough.

Include/Python-ast.h

-/* File automatically generated by ./Parser/asdl_c.py */
+/* File automatically generated by Parser/asdl_c.py */
 
 #include "asdl.h"
 

Parser/asdl_c.py

@@ -638,7 +638,11 @@ class ChainOfVisitors:
             v.emit("", 0)
 
 def main(srcfile):
-    auto_gen_msg = '/* File automatically generated by %s */\n' % sys.argv[0]
+    argv0 = sys.argv[0]
+    index = argv0.find('/')
+    if index > 0:
+        argv0 = argv0[index + 1:]
+    auto_gen_msg = '/* File automatically generated by %s */\n' % argv0
     mod = asdl.parse(srcfile)
     if not asdl.check(mod):
         sys.exit(1)

Python/Python-ast.c

-/* File automatically generated by ./Parser/asdl_c.py */
+/* File automatically generated by Parser/asdl_c.py */
 
 #include "Python.h"
 #include "Python-ast.h"