Allow the '=' character in the -D option of sphinx-build.py

sphinx/cmdline.py

@@ -204,7 +204,7 @@ def main(argv):
     confoverrides = {}
     for val in opts.define:
         try:
-            key, val = val.split('=')
+            key, val = val.split('=', 1)
         except ValueError:
             print('Error: -D option argument must be in the form name=value.',
                   file=sys.stderr)