cmake: use gflags package config instead of a find module

This allows to avoid conflicts between third party gflags find modules floating
around. If a package provide a local version of gflags find module, clients
which locate glog will incorrectly use it. Another problem is a CMake error message
"cmake_policy PUSH without matching POP" in nested find_package calls. In summary,
we need to ensure to use the original gflags package config.