Bug 951352 - Disallow XPIDL_MODULE if XPIDL_SOURCES isn't defined; r=glandium

Once upon a time XPIDL_MODULE and MODULE were kind of the same and
MODULE got mass-converted into XPIDL_MODULE with the removal of MODULE
in bug 939044. Unfortunately, we forgot to add a check for XPIDL_MODULE
being defined if XPIDL_SOURCES isn't (a build config that makes no sense
and only results in cargo culting).

This patch corrects that oversight.

--HG--
extra : rebase_source : d609e2baa8d16df97c944bc9def26ad05527d937