Log Message:
ucon64: Fix build on macOS. Patch from Daniël Hörchner.

"There are several reasons not to enable libdiscmage, which is different
than saying that it is unnecessary. It appears it is also unnecessary, as it
has never gotten further than alpha status and certain functionality was
actively disabled many years ago, but I never got any feedback about it.

The presence of libdiscmage caused confusion for some users and I had
discovered some issues in its limited functionality, so for version 2.2.1 of
uCON64 I decided to change the default of the configure script to
--without-libdiscmage. I probably should have stated that I consider
libdiscmage deprecated, because several distribution channels responded by
adding --with-libdiscmage to their build specifications for uCON64 instead.
Among those is pkgsrc."