With better directory layout and more proper include
statements we can avoid several local modifications,
such as changing config.h for Windows Glog and the
ones related on pass-through statements in logging
headers in Glog.
This commit also makes unused functions not-a-warning
for external code.
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.
