Fixing Win32 build with SCons

[joevenzon]

From what I see, the main benefit of GLEW is to provide the GL extension function headers, and a transparent way of calling those extension functions in Windows.

Yes, that's exactly it. GLEW makes available all of the opengl functionality that your graphics drivers support, regardless of whether or not you have the latest headers and libraries. That's really helpful for Windows, because it's stuck back in OpenGL 1.1. On other platforms we have updated headers and libraries, although one benefit I see is if people on other platforms don't have the latest OpenGL headers/libs, GLEW will make the functionality available anyway (like it does on Windows). On the other hand, if they don't have the latest OpenGL headers/libs and don't know how to upgrade them, they probably aren't going to have GLEW or know how to install it.