build fixes :
- remove ptexViewer from the build on OSX (doesnt work)
- remove maya plugins from the build on OSX (until we have a compatible version available)

glViewer fixes :
- fix incorrect GLFW version check in #ifdefs (now done in cmake)
- fix default program GL version (downgraded to 1.5 where necessary)
- remove non core-profile GL calls (triggering errors)

this should allow OSX 10.7 builds to produce a functional, if restricted, glViewer.

fixes #111