Use portable way to compute timezone offset instead of timezone/daylight,
the former is a function on some BSDs and the latter doesn't even exist
in DragonFly. Fix PLIST on swi-prolog-packages to not use NetBSD ELFism.
Bump revision of swi-prolog-lite and swi-prolog-packages.
The 5.6.x release consolidates the 5.5.x development stream started in 2004.
Highlights:
* UNICODE character set support
* Unbounded integer arithmetic and rational numbers
* New and faster CHR release
* Added clp(r) constraints over reals
* Many enhancements to the web and notably sematic web libraries
* Much more ...
