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)