Add gtest.
Squashed 'library/gflags/' content from commit 07031f9

git-subtree-dir: library/gflags
git-subtree-split: 07031f9703556abe8b6962983f5ad79bad780c15
Merge commit 'f51a8fbefc05265871e404a8e5f87982e26c59f0' as 'library/gflags'