Make the gflags library optional in the CMake build
Make the gflags library optional in the CMake build

When detected, add the necessary defines to actually use it.

Change-Id: I540c3e11e480be8aaab154ad91ee08cdc52319de
Reviewed-on: https://chromium-review.googlesource.com/988432
Commit-Queue: Frank Barchard <52415189a3f67d375195cbbb6fdb1f1759498e57@chromium.org>
Reviewed-by: Frank Barchard <52415189a3f67d375195cbbb6fdb1f1759498e57@chromium.org>

lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv