Update Makefile

You don't have to specify which compiler you want to use in the `Makefile`. Instead, define the `CC` variable at compile time like so: `CC=clang make`. This will help you switching between compilers much easier. This will also not get in the way of people who want to compile your game but doesn't have clang installed.
Merge pull request #4 from grrk-bzzt/patch-4

Update Makefile