AUTOMOC is always rebuild

Currently if you have AUTOMOC turned on for a target then it means that always the moc files are being rebuild. This is counter productive as you don't want to rebuild them if non of the sources gets modified.

I have found #17161 (closed) where @sebholt mentions:

AUTOMOC is supposed to always rebuild because it checks files that are not listed in the sources (like extra "_p.h" headers) and which may change outside the visibility of the generator. I'm not sure if the behavior can be changed for AUTOMOC without breaking existing projects.

There should be an escape hatch for people who are nicely declaring all relevant sources to the generator and are not relying on the glob-like way of obtaining all possible inputs.