Workaround for "Not found: /data/art/mouse.png"

http://dwarffortresswiki.org/index.php/DF2014:Installation#.22Not_found:_.2Fdata.2Fart.2Fmouse.png.22_and_similar_errors
This patch will force DF to use system zlib (if available) and not one included with the game.

Tested on Arch Linux,