Fixed so 'get_source_files()' calls 'check_extension_list()' -- that way,

we can run "sdist" on a distribution with old-style extension structures
even if we haven't built it yet.  Bug spotted by Harry Gebel.

command/build_ext.py

@@ -309,7 +309,7 @@ class build_ext (Command):
 
 
     def get_source_files (self):
-
+        self.check_extension_list()
         filenames = []
 
         # Wouldn't it be neat if we knew the names of header files too...