When adding a new header extension with
QMAKE_EXT_H += .myh
The moc compiler generates a moc_.cpp file *without the necessary #include to the original .myh file.
Later on, the compilation of the moc_*.cpp file fails with "not declared" errors, of course.
I'm attaching a tgz with the simplest case to reproduce the error.
It works as it is, changing the .pro file commenting the usual .h an uncommenting the lines to enable .myh leads to the reported error.