Added Linux Support

I created a makefile as well as small improvements to the code that allows for compilation on Linux. Additionally, I created a tarball of the executable and game assets for Linux [here](https://github.com/tomgalvin594/SFML-Gamejam2/releases/tag/v1.0) (specifically [this link](https://github.com/tomgalvin594/SFML-Gamejam2/releases/download/v1.0/SFML-Gamejam2.tar.gz)).

Good job on the game, especially considering you had less than 15 hours. Hope to see you next jam. :smiley: