fontconfig-ultimate: 2015-12-06 -> 2016-04-23

This removes our hardcoded presets which weren't updated for quite some time.
Infinality now has new hardcoded presets in freetype, which can be overriden if
desired with environment variables (as before). Accordingly, updated NixOS
module to set the hardcoded preset.

Additionally used a more "right" type for substitutions.