(ITS#9024) [PATCH] Fix union semun undefined from FreeBSD 12 onward

[khng300@gmail.com]

Full_Name: Ka Ho Ng
Version: mdb.master
OS: FreeBSD 12.0-RELEASE
URL: ftp://ftp.openldap.org/incoming/Ka-Ho-Ng-190517.patch
Submission from: (NULL) (2001:470:fa95:1300::3)


Starting from __FreeBSD_version 1200059 union semun definition was removed
from userspace headers in order to comply with POSIX. In order to resurrect
the defintion we need to define _WANT_SEMUN before including sys/sem.h for
__FreeBSD_version >= 1200059.

I tried to touch as small amount of code as possible here to avoid interfering
with other platforms I do not use.

By the way, please help close ITS#8986 as this version is a resend of the patch.