Added files for Linux version.

Added main.cpp to be the entry point for the Linux version of the
application. Also added a Makefile for building on Linux.
Made program build on Linux.

This involved three primary changes. First, use ifdef
to select the properly OpenGL include statements. Second, use ifdef
to switch between different Box2D version on Linux and Apple. Finally
make some changes to handle the weaker video card on my Linux laptop.