swi-prolog*: Update lang/swi-prolog* to 7.6.4

pkgsrc changes:
- Add patches/patch-src_os_pl-files.c to avoid overlapping `src'
  and `dst' in srtcpy(3).  It is an undefined behaviour and it leads to build
  issue in swi-prolog-packages on platforms that uses SSP.
- readline dependency is no longer needed for swi-prolog-lite.
  The readline functionality is now provided as a package via
  swi-prolog-packages.
- Inject CFLAGS via COFLAGS and pass -fPIC via CMFLAGS so that
  swi-prolog-jpl will link
- Add patches/patch-src_pl-rsort.c to use FreeBSD qsort_r(3)
  instead of using nested functions. This is probably more portable
  and has less problems (e.g. does not need for PaX MPROTECT).

Changes:
7.6.x
-----
Major release. Multi-argument indexing, better multi-threaded
performance, threaded global GC (atoms and clauses), mode-directed
tabling, restored same expand and directive semantics for initial
load and reloading files, cleaner initializatin and command line
handling.

7.4.x
-----
Major release. More robust and better scalable support for multi-core
hardware, tabling, engines. Many enhancements to the RDF, HTTP and
SSL libraries. This is the first release licensed under the Simplified
BSD (BSD-2) license.

7.2.x
-----
Major release, providing native key-value support by means of dicts,
native strings using "hello world" syntax, web-accessible Prolog
engines (Pengines) and a high level SQL library called CQL.

Please note that this is just a summary of the most important changes,
for more information please give a look to:

<http://www.swi-prolog.org/ChangeLog?branch=stable>

(leot)