[gentoo-desktop] First steps into X11 global multilib

[Michał Górny]

Hello,

A quick patch to xorg-2.eclass to support enabling multilib on packages
and a few example conversions to prove the point. I'd some feedback
before I continue with the packages.

In order to test those packages, you need a few patches to the gx86
eclasses, namely:

- http://article.gmane.org/gmane.linux.gentoo.devel/82764
- http://article.gmane.org/gmane.linux.gentoo.devel/83083 (optional)
- https://453206.bugs.gentoo.org/attachment.cgi?id=336232

I had to move two xproto .pc files to /usr/share/pkgconfig to get them
found by configure scripts. I checked the relevant packages whether
the installed includes are the same for amd64+x86 but I don't know if
there can be a case when they are different.

If there is, we will have to make the packages support multilib and use
a custom header install location on other ABIs. In any case, it can
be fixed in the future.