autotools: error out when building with mangling and glvnd
It's not a thing that can work, nor is a wise idea to attempt. v2: Tweak error message (Dylan) CC: <mesa-stable@lists.freedesktop.org> Signed-off-by:Emil Velikov <emil.velikov@collabora.com> Reviewed-by: Adam Jackson <ajax@redhat.com> (v1) (cherry picked from commit 25a9450a)