Currently the base.txz distribution does not get the BSD.debug.dist mtree
extracted into it. So if you start from that and then try to build a 3rd-party
application outside of buildworld it will by-default try installing the
debug files into a missing directory if they are being installed into /usr/lib.
Check for the existence before forcing the directory to be created rather than
the older way of running a shell command with test -d || mkdir -p always.
Reported by: HardenedBSD (https://github.com/HardenedBSD/secadm/issues/23)
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D5411
a324b0f04c