Version 12 08 branch (fixes for bugs 203 and 259)

Since Mercury appeared at github I'm back on making Exherbo package for it. I've managed to make patches for specific branch to get it compiled and installed from scratch (no bootstrapping).
This should at least partially fix [bug 203 related with DESTDIR](http://bugs.mercury.csse.unimelb.edu.au/view.php?id=203) and [bug 259 related with libdir and others](http://bugs.mercury.csse.unimelb.edu.au/view.php?id=259).
Please consider pulling these changes in.

Thank you