Make Argument Clinic's '--make' option work on Windows.

Tools/clinic/clinic.py

@@ -4164,7 +4164,7 @@ def main(argv):
             cmdline.print_usage()
             sys.exit(-1)
         for root, dirs, files in os.walk('.'):
-            for rcs_dir in ('.svn', '.git', '.hg', 'build'):
+            for rcs_dir in ('.svn', '.git', '.hg', 'build', 'externals'):
                 if rcs_dir in dirs:
                     dirs.remove(rcs_dir)
             for filename in files: