#178 location format for gcc not useful for hierarchial source

The comments in CompilerOutputter.h suggest that "%f:%l:" is the default for gcc compilers. This isn't useful for the common use case of this outputter -- stepping through to the next error -- when you're building unit tests in a source tree with many directories.

e.g.

foo/module1/code.cc
foo/module1/code_test.cc

foo/module2/code.cc
foo/module2/code_test.cc

with the default of "%f:%l:" you won't know which test an error occurred in, and emacs won't know where to find it.

an explicit

outputter.setLocationFormat("%p:%l:")

does the right thing, but this location format should be the default.

Thanks!