Bug 1229241 - Allow moz.build special variables to be set, as long as the value is not modified. r=gps

This will allow a new kind of special variable where it is possible to do
  FOO += ['bar']

All the current special variables are either strings (for which __setitem__ would
be called with a different string object), or a read-only dict (which doesn't
allow modifications).

python/mozbuild/mozbuild/frontend/reader.py

@@ -219,6 +219,8 @@ class MozbuildSandbox(Sandbox):
         return Sandbox.__contains__(self, key)
 
     def __setitem__(self, key, value):
+        if key in self.special_variables and value is self[key]:
+            return
         if key in self.special_variables or key in self.functions or key in self.subcontext_types:
             raise KeyError('Cannot set "%s" because it is a reserved keyword'
                            % key)