wafsamba: Only use $DESTDIR in INSTALL_DIR() if it is set

wafsamba: Only use $DESTDIR in INSTALL_DIR() if it is set

Otherwise the leading '/' is stripped and directories are created
relative to the current directory.

This fixes a regression introduced in recent commit
26ea0f58daace4adef7c5bb17f19476083bf3b7b.

Reported-by: Ralph Böhme <slow@samba.org>
Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
Autobuild-User(master): Martin Schwenke <martins@samba.org>
Autobuild-Date(master): Thu Sep 13 09:37:23 CEST 2018 on sn-devel-144