QtAutogen: Do not re-generate AUTORCC outputs on every build

Refactoring in commit v3.6.0-rc1~134^2~10 (Autogen: Split out moc file
generation code to dedicated method, 2016-04-18) removed the
unconditional creation of the `<target>_automoc.cpp` file.  Now it is
generated only when `AUTOMOC` is enabled.  However, if this file is not
created then our internal `GenerateAll` setting is enabled on every
build, causing `AUTORCC` to re-generate its file(s) every time.  Fix the
`GenerateAll` setting to be used only for when autogen settings change.
The old logic was left from when we had only automoc.

Closes: #16413