cmake: Check that gbm is not enabled on Mac
cmake: Check that gbm is not enabled on Mac

Checks for GLX, EGL, and Wayland were present. GBM was the only check
missing.

Signed-off-by: Chad Versace <a557f7f75bf28dc19ac99107f5cc7cd6b3fbdb1a@linux.intel.com>

evelikov/waffle,fjhenigman/waffle,dcbaker/waffle,maurossi/waffle,evelikov/waffle,dcbaker/waffle,maurossi/waffle,janesma/waffle,fjhenigman/waffle,waffle-gl/waffle,waffle-gl/waffle,waffle-gl/waffle,janesma/waffle,yinquan529/waffle-gl-waffle,fjhenigman/waffle,janesma/waffle,chadversary/waffle,chadversary/waffle,waffle-gl/waffle,maurossi/waffle,yinquan529/waffle-gl-waffle,yinquan529/waffle-gl-waffle,chadversary/waffle,dcbaker/waffle,evelikov/waffle