Restrict download htmlxref.cnf from my LOCAL

Do not download htmlxref.cnf from GNU directly to avoid checksum error when
upstream updates this unversioned file.

> htmlxref.cnf doesn't seem to exist in /usr/ports/distfiles/texinfo/6.5.

> Attempting to fetch http://ftp.gnu.org/gnu/texinfo/htmlxref.cnf

fetch: http://ftp.gnu.org/gnu/texinfo/htmlxref.cnf: size mismatch: expected 20137, actual 20118

MFH: 2017Q4