Blender Git Loki

Merge branch 'master' into soc-2019-openxr

Linux: OpenXR Linking via OpenXR Loader

Calling OpenXR functions should now work on both Windows and Linux.

Also includes:
* Silence Linux only warnings
* Remove common_cmake_config.h and let CMake generate it when
configuring (contents vary on system it's compiled on)
* Remove JsonCpp CMake install target
* Remove unnecessary CMakeLists for JsonCpp includes
* Remove unnecessary CMake commands
* Style cleanup

Windows: OpenXR Linking via OpenXR Loader

Adds needed headers for OpenXR, the loader from the OpenXR SDK and
JsonCpp into extern. Took a while to get this to compile/link, but on
Win10 it works fine now without patching #includes. Linux probably
needs more work.

Added a compile option WITH_OPENXR to toggle XR feature compiling.

Also does a dummy xrCreateInstance() call to test linking.