Updated cross-compile instructions to fix missing libintl.h

Short commit, long story.

I tried to cross-compile from Linux to Windows using the instructions in COMPILING.md, but the gcc built by mxe couldn't find libintl.h, a dependency that came up recently due to the translation effort.

libintl is part of gettext which is part of glib, so the best thing to do is to have mxe also build glib so that it has libintl.a available at compile-time.

Having gotten this working, I figured it would be best to update the cross-compile instructions for anyone having a similar problem.

(On a related note, the cross-compile was done on OpenSuse 12.3, which means that it should work on Fedora too, in addition to the already-confirmed Ubuntu. Since I added the required packages through YaST's package manager, I have nothing to add to the installation instructions, only mxe's build command)