Removed IECoreGL dependency on GLUT.

We were only using it to create a context suitable for running the tests and other standalone programs, and it always popped up a window annoyingly.

This is the first of a series of pull requests where I'll be bringing stuff over from the coreGLMac branch until we have master building on both Mac and Linux again. The history on coreGLMac is a mess due to me stumbling around in the dark learning git and inconvenient truths about OpenGL on OSX all at the same time. So I'll be making new, more sensible, commits rather than making one big merge request.