automoc by itself can only detect metadata files referenced by direct
usage of the Q_PLUGIN_METADATA macro. It does not do any C++ preprocessor
evaluation. Instead it needs to be helped with regexp-based filter rules
for detecting any names of files used as additional moc input.
See docs for AUTOMOC_DEPEND_FILTERS for further details.
In the near future all the boilerplate code should be replaced with a yet to
be designed ECM macro. For now explicit code is used to collect use cases
for the macro and still to fix the issue already.