Support gcc coverage

I wrote a module that adds "–coverage" to compiler flags to use GCC's coverage feature (with lcov etc). Cleaning the project does not delete the generated *.gcda and *.gcno files. These are obviously not lists as output artifacts in the CPP.compilerOutputArtifacts() method in cpp.js.

On the next run of the instrumented unit tests libgcov complains about inconsistent build ("overwriting an existing profile data with a different timestamp").

It would be create to get support for this.

Thank you!