Fall back to building the package if prefer_system_check fails

[singiamtel]

The current behaviour is to crash the build, but we can recover
gracefully by going back to building our own version instead.

Any trigger of this warning should be considered as a bug on the recipe,
though

[singiamtel]

@ktf what should I do with this test? Feel awkward to just delete it, but I'm not sure what to test for with the new behaviour change

    def test_missing_replacement_spec(self, mock_dieOnError):
        """Check an error is thrown when the replacement spec is not found."""