For Mac OS X, let's now use the default X11 locations for the fonts.conf and the cache files.
Those that do not have X11 installed (old default < 10.4) are simply out of luck.
They will only be able to use the files in /System/Library/Fonts and will need to rebuild
their cache every time.

We should also fix this for windows. I'm looking for other projects that use fontconfig on
Windows to see if I can get some hints on how to do this.