Libmv: Solve some strict compiler warnings

This commit is to be backported to upstream still

Move Libmv from extern/ to intern/

Logically it is intern library since being mainly developed by 1.5 blender guys.

Move Ceres to extern/

Even tho it's currently only used by Libmv we might use it for something
else in the future. Plus, it's actually where it logically belongs to.

Move GLog/GFlags to extern/

This is where the libraries belongs to actually, they are not only used by
Libmv now, but also by tests and Cycles.

Re-organize structure of GLog/GFlags CMake libraries

The idea is to split them into two separate targets and have dedicated include
directories list for each of them in order to avoid some annoying include header
modifications in comparison with upstream.

Reviewers: campbellbarton, juicyfruit

Differential Revision: https://developer.blender.org/D1706