Fix MacOS X dylib versioning

Merged patch to correct MacOS X dylib versioning. (via Douglas Carmichael)

  From: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
  Date: January 12, 2014 at 5:07:00 PM CST
  To: Douglas Carmichael <dcarmich@dcarmichael.net>
  Subject: Re: [115835] trunk/dports/audio/xmp/Portfile

  The library filename and dylib should only change if the new library is not
  backwards compatible with code linked against the old one.

  Ideally:

  libxmp.dylib should be a symlink to libxmp.4.dylib
  libxmp.4.dylib should be the real filename with a dylib id of
  ${prefix}/lib/libxmp.4.dylib
  libxmp.4.dylib should have a valid compatibility version and current
  version (probably compat = 4.0.0 and current = 4.2.2 in this case).

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Update changelog

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>