Blender Git Loki

[cmake/cpack] allow override of package name

Cpack generates a standard filename with git information in it, which might not always be wanted for release builds, this patch adds an option to override that default filename.

Reviewers: sergey, juicyfruit

Reviewed By: juicyfruit

Differential Revision: https://developer.blender.org/D2199

ammended to fix: wrong variable name in main CMakeLists.txt