util: unbreak endian detection on OpenBSD

util: unbreak endian detection on OpenBSD

Since cbee1bfb34274668a05995b9d4c78ddec9e5ea4c endian.h is unconditionally
used if available.

glibc has byte order defines with two leading underscores.  OpenBSD
has private defines with a single leading underscore in machine/endian.h
and public defines in endian.h with no underscore.

The code under the endian.h block did not check if symbols were
defined before equating them so '#if __BYTE_ORDER == __LITTLE_ENDIAN'
would turn into '#if 0 == 0' which is always true.

Fixes: cbee1bfb342 ("meson/configure: detect endian.h instead of trying to guess when it's available")
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5630>