GPU: add BUIDTIME to WITH_GPU_SHADER_BUILDER

Adds a better name that describes when it is used.
The GPU_SHADER_BUILDER is a buildtime tool for developers
to pre-validate GLSL (and in the overseen future pre-compile to
SpirV). We don't see that this needs to become a required
step in the future so WITH_GPU_BUILDTIME_SHADER_BUILDER
is more descriptive name.