New USES macros that loads "features" on demand.

First feature implemented: pathfix which will lookup in WRKSRC for Makefile.in
and configure and fix common path that doesn't fit with FreeBSD hier(7)

This is an extract of bsd.gnome.mk's gnomehack to avoid porters to have to load
the full bsd.gnome.mk just for this feature

More features to come...
import gettext.mk and bison.mk from the master which are required by sysutils/grub2
Merge pull request #72 from reallyenglish/mitsuruy

introduce USES feature to the tree, which is required by sysutils/grub2