CMake build doesn't regenerate mocs on configuration changes

I suppose this is a cmake bug, but since cadam will most likely handle it anyway, I'll report it here

I built Qt Creator in Release mode. The build was finished successfully.

Then I changed the configuration and added WITH_TESTS. Most of the files were rebuilt, but the moc files weren't, so classes that are declared in #ifdef WITH_TESTS like Core::TestVersionControl did not have moc, then it failed to link.