build ROOT with builtin GLEW

When I tried to install ROOT 5.34.07 on Mountain Lion (10.8.3), I got this error:

    Undefined symbols for architecture x86_64:
      "___GLXEW_ARB_multisample", referenced from:
          TGLFormat::InitAvailableSamples()      in TGLFormat.o
    ld: symbol(s) not found for architecture x86_64

Following the advice from a Macports user [here](https://sft.its.cern.ch/jira/browse/ROOT-4417), I added `--enable-builtin-glew` and got it to build. I also tried the OpenGL rose demo from [this thread](http://root.cern.ch/phpBB3/viewtopic.php?t=12148&p=52494) so I think it actually works.