Jonathan Gray authored Mar 28, 2018 and Eric Engestrom committed Sep 02, 2020

Since cbee1bfb 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: cbee1bfb

 ("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: <mesa!5630>
(cherry picked from commit 7eab6845)