libgl-gdi: support building without softpipe

While we do need *some* fallback-driver, there's no good reason to
*always* require that to be Softpipe. LLVMpipe for instance does the job
just fine.

This makes the minimal build a bit smaller on Windows.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7506>
2 files changed