Fix Mac build.
Fix Mac build.

OSX 10.10.5 (Yosemite) doesn't support OpenGL3.1.

Reverting to previous OpenGL2.1 for OSX.

Change-Id: I8533c2e52233e057603ffb2809650f87f1c2266f
Tested: Mac build now works.

google/fplbase,google/fplbase,google/fplbase,google/fplbase