Blender Git Loki

Fix install paths for blender thumbnailer when not building a portable install

When doing a non portable build of blender, the executable
blender-thumbnailer would be installed in two locations:
/usr/bin/
/usr/

While cleaning up, also make the blender thumbnailer dll optional on
windows to bring the logic in line with what it is on linux and mac.

Reviewed By: Campbell Barton, Ray molenkamp

Differential Revision: https://developer.blender.org/D13014