Fix compilation error introduced by 130deabb4. See #449

Extension of the "moc_" file will either be cpp or cxx.

Note that to be compliant with Qt [1], for Q_OBJECT class declarations in implementation
(.cpp) file, a file with ".moc" extension should be generated.

[1] http://qt-project.org/doc/qt-4.8/moc.html
Merge branch '449-speedup-cmake-config-and-generation'

* 449-speedup-cmake-config-and-generation:
  Fix compilation error introduced by 130deabb4. See #449