configure: don't require libudev for gbm or egl drm/wayland

After the loader changes libudev is no longer required for
gbm or the egl drm/wayland platforms.  Lets these build/run
on OpenBSD.

v2: preserve the libudev requirement for Linux as suggested
by Emil Velikov.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
1 file changed