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)