Set gcc/g++ version

[John-Philip Johansson]

Use the same gcc and g++ version as RetroPie.

Currently that's 4.7 and we probably want something like this:

cmake -D CMAKE_C_COMPILER=gcc-4.7 -D CMAKE_CXX_COMPILER=g++-4.7 .

Need to make sure we have that installed on our machines, and OSX currently comes with 4.2 so we need to look into how to make this work.

References:
http://jamesreubenknowles.com/how-to-install-gcc-4-7-on-mac-os-x-1774