using ::gflags seems incompatible with homebrew's gflags 2.0

Changing the namespace to `::google` fixed the issue for me.

Also the header file being used (`gflags/gflags.h`) seems a bit outdated as well, since by default homebrew puts `gflags.h` under `google/gflags.h` (this seems to confirm that namespace `::gflags` has been merged into `::google`).