We currently have two checks (that are supposed to have the same
result) in the changed if-stmt and a few lines above where we
set cpp_module_file. Obviously if we generate a module, we should
also set cpp_module_file, however (due to the duplicated code) in
the case of multidicts we generate a module without specifying the
file. This probably causes CMake to behave incorrectly when rebuilding
PCM files.