GLEW support for OpenGL 3+ core profiles

As the core profile fix of in-tree GLEW is rather hackish, I tried to ameliorate the situation by pulling in the suggested patch to http://sourceforge.net/p/glew/patches/40/ and refactoring it a bit.

The new approach is to store reported extension names in an open hash set. My improvement is to use the hash set even for OpenGL environments before v3.0 instead of performing a linear search rhough the complete extension string on every lookup.