Fake Windows [For testing]

With this patch, `./configure --enable-{fake,win}gdi` allow you
to use the wingdi method under Linux, if RandR is enabled, it
will be used, if not enabled the implemention does nothing.

The multimonitor support suggested in issue #59 is supported
by this compatibility layer.

The idea with the patch is to make it easy to keep gamma-w32gdi
up to date with changes in the other adjustment methods.