OK, here's a new patch. OK to apply it?

This actually also fixes the existing problem of all bootstrap objects
being rebuilt of eval.scm is touched.

The patch is verified to give a faster build for 4 and 32 build threads.

The only downside is that it requires GNU Make 3.8 (which was released
2002) or later, but that shouldn't be a problem, right?

Best regards,
Mikael