moc's don't compile when using a custom library with cmake

When creating a CMake QT library using qt_add_library() mocs don't seem to be built properly.

I came to this conclusion because I got vtable errors in the linking stage trying to statically build qwt and use it in one of my projects. The mocs seem to generate correctly (in mocs/include) but don't get included in mocs_compilation.cpp so they don't get compiled and cause the project to not link correctly.

qwt with cmake I used: https://github.com/Sani7/qwt

EDIT:

minimum reproducer: https://github.com/Sani7/qwt_cmake_error

Error in Qt creator

(I only used a thermo in my example, it gives more errors if you have more qwt widgets.