mesa: setting meson opt mesa-clc to 'auto' if LLVM support disabled

mesa: setting meson opt mesa-clc to 'auto' if LLVM support disabled

mesa-clc set to 'system' requires the system tool 'mesa-clc' which gets
built as part of the mesa/host build.

The mesa/host build however depends on LLVM and others - hence we're
trying to avoid pulling this in where possible if target configuration
allows.

meson option 'mesa-clc' allows for 3 options: enabled, system, auto.

While 'auto' sounds like it will only automagically go for either
'enabled' or 'system', it will also disable CLC support entirely, if
only drivers are enabled which don't need it (resulting in
`with_clc=false`) - which is exactly what we want.

This relies on only drivers being selectable, which don't automatically
enable CLC - if LLVM support is disabled.