Custom PREFIX + USE_LDCONFIG=yes creates leftovers

[Raphael Kubo da Costa]

I've been testing cad/brlcad as part of my work to update CMake to
3.1.0. One thing I noticed with "poudriere bulk -t" (regardless of the
CMake version) is that it fails at the end with leftovers:

====>> Error: Files or directories left over:
@dir /usr/local/libdata/ldconfig

This seems to be caused by brlcad's use of a custom PREFIX,

PREFIX?=	${LOCALBASE}/${PORTNAME}
NO_MTREE=	yes

since USE_LDCONFIG causes a file to be created outside the PREFIX.

USE_LDCONFIG's documentation in bsd.port.mk indicates it's supposed to
write its files to ${PREFIX} instead of ${LOCALBASE}, but r357076 says
otherwise.

What's the right way to fix this?