'Decide which souce files must be recompiled.
Determine the list of object files corresponding to \'sources\',
and figure out which ones really need to be recompiled.
Return a list of all object files and a dictionary telling
which source files can be skipped.'

def _prep_compile(self, sources, output_dir, depends=None):

    objects = self.object_filenames(sources, output_dir=output_dir)
    assert (len(objects) == len(sources))
    return (objects, {})
