Change gflag.cmake to use default repo
Created by: wangkuiyi
Currently, our gflags.cmake file refers to my personal fork of gflags:
This is because of a bug in the official gflags repo as mentioned in https://github.com/PaddlePaddle/Paddle/issues/3277. I filed a PR https://github.com/gflags/gflags/pull/230 to fix the bug, and it seems that gflags maintainers have merged it. So it is the time for us to switch back using the official gflags repo.