Remove -fopenmp compiler flag

OpenMP is not used in OpenGL examples.
Prevents errors on OS X, because Xcode 5 does not bundle GCC anymore, and Clang compiler does not support OpenMP yet.

See: 
http://stackoverflow.com/a/13720279/1716747
http://stackoverflow.com/a/17164844/1716747

Also, OpenMP will be supported in Clang/Xcode eventually, but it is not merged yet:
http://clang-omp.github.io/