Update unpack_template.h
- [x ] closes #14373
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

USE_CASE_RANGE is a GNU C feature. This change will activate
USE_CASE_RANGE on any platform when using GNU C and not on any platform
when a different compiler is being used.

closes #14373
