A few changes were made to Ogre's copy of GLEW. It would be better to change the code to work with a stock version of GLEW. I know this is possible, just not sure yet how to do it cleanly.
The advantages we would get:
GLEW can be swapped out with a newer version if required.
Smaller OGRE libraries
glew.cpp triggers tons of compiler warnings when built with Ogre's compile settings.