* Add separate FindUSDPixar.cmake and FindUSDHoudini.cmake for building
  against two types of USD libraries.
* Add separate BUILDING.md including Hydra build instructions.
* Hide symbols on Linux and macOS to avoid library version conflicts.
* Override OpenColorIO lib to share configuration with host application.

Thanks Patrick for helping with the Windows side of this.

Differential Revision: https://developer.blender.org/D14594