Linux version now compiled on Ubuntu 12.04

I updated the Linux version once again. It is now compiled on an Ubuntu 12.04 version. Apparently Ubuntu 14.04 which is the oldest supported version of Ubuntu has major problems with SDL2.

Compiling on Ubuntu 12.04 solves a very annoying problem where the game would crash when changing to fullscreen on Linux.

Keeping an unsupported OS to compile is very annoying. It is my goal to always support the oldest LTS version of Ubuntu. It will properly be necessary to compile on Ubuntu 12.04 as long as Ubuntu 14.04 is the oldest support version of Ubuntu.