bugfix: Fix fontconfig /etc/fonts default

from http://lists.science.uu.nl/pipermail/nix-dev/2013-June/011303.html

> The problem is that fontconfig has renamed thier --with-confdir
> configuration option to --with-baseconfigdir.

> Can you or someone else patch
> pkgs/development/libraries/fontconfig/default.nix ?

Fixes #620