Bug 928204 - Don't ignore missing depfiles when aggregating them. r=gps
Bug 929896 - Make GTEST_CPP_SOURCES a StrictOrderingOnAppendList. r=mshal
Bug 913268 - Make CPP_SOURCES a StrictOrderingOnAppendList. r=mshal
Bug 921492 - Make StrictOrderingOnAppendList use actual alphabetical sorting. r=mshal,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 930227 - Don't use lists to link host binaries. r=gps

Host and target linkers may not support the same kind of lists expandlibs uses.
On the other hand, lists are only really needed for libxul, where the command
line length or number of arguments may overflow the kernel support. So just
don't use them for host linkage.