Before you compile, make sure you have the GLEW libraries. On Mac OS X:

brew install glew

After you've installed them, just run

make pong

Then, ./pong, and behold:

• Windows (props to @code_head for this)
• OS X

Top player moves left / right with the 'a' and 'd' keys, respectively, while the bottom player moves with the arrow keys. If the ball breaks through your paddle, the other player gets a point. The game ends whenever you want it to end :).