It is disabled by default, so should not affect existing configurations.
Main benefits of this goes as:
- Linux distros can use that to avoid libraries duplication and link
blender package against gflags package from the system.
- It it easier to test whether Blender works with updated version of
Gflags prior to re-bundling the library.
Main purpose of this is to bring new gflags library which is more likely
to have a fix for undefined order of static variables initialization and
also to bring new glog where some compilation error are fixed (which are
only visible with more strict checks with clang and c++11 enabled).
