Reduce OpenMW's compilation time
It would be nice to reduce OpenMW's compilation time:
- travis-ci timeouts from times to times
- coverity's scan timeout, so we have to restrict ourselves to openmw only
- compiling with custom instrumentation for fuzzing might significantly increase compilation times
- not everyone has a fancy computer, and it's annoying to wait more than half an hour for a compilation on a 10 years old CPU.