gwenhywfar: 4.11.1 -> 4.15.3

Upstream changelog for version 4.11 through 4.12 doesn't seem to be
available anymore and the change summaries from the project page are in
German, but here's a comparison from a GitHub mirror I've found:

https://github.com/cstim/gwenhywfar/compare/4.11.1beta...4055683538b6

I've shortened the URL to not include the file name in the URL itself
because we use the name attribute in fetchurl to force the name anyway.

A quick way to find out about the upstream download URLs is:

curl 'http://www.aquamaniac.de/sites/download/packages.php?package=01&showall=1

' \
  | sed -nre 's/^.*<a\>.*\<href="([^"]+)".*/\1/p'

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @cillianderoiste, @urkud