Update CMake build to support gflags and gtest correctly
Update CMake build to support gflags and gtest correctly

The gflags library was missing from the CMake build
and the gtest includes were not being set correctly.

R=fbarchard@google.com

Review URL: https://codereview.chromium.org/1692083002 .

lemenkov/libyuv,lemenkov/libyuv,yarrcc/libyuv-ios,yarrcc/libyuv-ios,lemenkov/libyuv,yarrcc/libyuv-ios,lemenkov/libyuv,yarrcc/libyuv-ios