Bug 930899 - Improve backend file accounting. r=gps
Bug 930896 - Keep track of files generated by a build backend. r=gps
Bug 928364 - Remove unified source dependencies completely, they're useless. r=gps

The backend.RecursiveMakeBackend.built business ensures config.status reruns
when moz.build changes, which is the only thing that would affect unified
sources. Rerunning config.status updates unified sources if they need to
be updated. Once updated, normal object dependencies will ensure they are
rebuilt. Those same dependencies will ensure they are rebuilt when one of
the C/C++ sources they include change. IOW, unified sources don't need any
explicit dependencies generated by the backend.
Bug 928244 - Add a UNIFIED_SOURCES variable to moz.build to trigger unified builds of the given sources. r=gps