Blender Git Loki

CMake: add WITH_UNITY_BUILD option to improve compile times

Unity builds are only used in the `bf_nodes_geometry` module for now.
This module has been prepared to support unity builds already.
Usually, there is a 2-4x speedup when building `bf_nodes_geometry`
compared to without unity builds (e.g. 145s to 55s).

For more information about how unity builds work and how they help
with compile times, see D13341.

Differential Revision: https://developer.blender.org/D13341