Return the default build value to use when declaring a main target,
            which is obtained by using specified value if not empty and parent's
            default build attribute otherwise.
            specification:  Default build explicitly specified for a main target
            project:        Project where the main target is to be declared

def main_target_default_build (self, specification, project):
        """ Return the default build value to use when declaring a main target,
            which is obtained by using specified value if not empty and parent's
            default build attribute otherwise.
            specification:  Default build explicitly specified for a main target
            project:        Project where the main target is to be declared
        """
        assert is_iterable_typed(specification, basestring)
        assert isinstance(project, ProjectTarget)
        if specification:
            return property_set.create_with_validation(specification)
        else:
            return project.get ('default-build')