let GLEW report modern OpenGL versions/extensions

Without enabling this option, in my system (Ubuntu 14.04 with the most recent nVidia drivers) some functions like `glGenVertexArrays` will be NULL even after `glewInit` returns successfully.  This causes no compiler errors; programs will just crash at runtime.

This ever helpful blog post claims that "By default, GLEW is only configured to make use of features available in OpenGL 2.0 and lower." (!!!)

http://www.brandonfoltz.com/2013/12/example-using-opengl-3-0-with-sdl2-and-glew/