Add a comment in util/Makefile for how to enable
OpenMP for MacOS.  Requires updated gcc or clang compile.

Bug: None
Test: /usr/local/bin/g++-7 -msse2 -O3 -fopenmp -static-libgcc -o psnr_omp psnr.cc ssim.cc psnr_main.cc
Change-Id: Icb3389bf8cf94f09a185fea055c69823b9fbc66b
time ./psnr_omp  -ssim -s 1920 1080 ~/test/garden2_mp4.yuv ~/test/garden2_ogv.yuv
Reviewed-on: https://chromium-review.googlesource.com/807546Reviewed-by: Weiyong Yao <braveyao@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>