automoc: moc_compilation.cpp is regenerated for changes in source files that don’t need it

In a project with CMAKE_AUTOMOC ON in addition to all the header files, all source files seem to get processed by AUTOMOC. So far so good if the source file actually requires it. However, it seems like modification of any source file now requires recompilation of moc_compilation.cpp (because that gets regenerated – with no changes) severely impacting incremental build times. iirc this is a recent change