The qmake build system has accumulated a bit of cruft and redundancy over time. There's also a fairly tight coupling between how to build the various targets, and _what_ to build, making it harder to add new rules or sources.
Patch landed in r99274. Please block any fallout on this bug and I'll close it once we've stabilized things again.
I'm updating the SL bot as we speak.
Closing this, now that the major changes have landed.