This fixes the CI/licensing issues introduced by 68375667 without
regressing #67. We will now link against SPIRV-Tools where we can, and
won't otherwise. Apparently glslang can sometimes depend on SPIRV-Tools
and sometimes not...

A better solution for this might still be needed to avoid licensing
issues. But for now, I don't particularly care, especially since we
don't actually use any code from SPIRV-Tools so in theory the license
shouldn't matter.