Allow projects based on the FreeBSD tree to append to _PRIVATELIBS
and _INTERNALLIBS by simply maintaining their own lists of
LOCAL_PRIVATELIBS and LOCAL_INTERNALLIBS, respectively.

Reviewed by:	bdrewery
MFC after:	1 week
Sponsored by:	Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D33901