Make glslc program really required when Vulkan is used

The REQUIRED keyword has to be uppercase to stop the cmake step if glslc is not found.

approved this merge request

assigned to @pswartva

requested review from @faverge

added 5 commits

• 231f5f68...df18a58d - 4 commits from branch solverstack:master
• ef2f941f - Make glslc program really required when Vulkan is used

Compare with previous version

enabled an automatic merge when the pipeline for ef2f941f succeeds

merged

mentioned in commit d0676160