Using unordered_map and unordered_set C++ container types currently
requires careful testing or usage of boost, due to the various confusing
C++ version differences in include paths and namespaces.
Libmv defines tests for these cases in cmake and scons, such that ceres
can use any available implementation, or fall back too std::map/std::set
if none can be found.
This patch generalizes this buildfile code by providing a Blender macro.
* cmake: defines both the variables used by libmv at them moment as well
as 2 variables UNORDERED_MAP_INCLUDE_PREFIX and UNORDERED_MAP_NAMESPACE,
which can later be used in other C++ parts for convenience.
* scons: adds a tool script returning the include prefix and namespace.
Libmv checks these to define the appropriate definitions for ceres.
Differential Revision: https://developer.blender.org/D425
