sdl-perl 1.20.3: Fix the build of sdl-perl

sdl-perl 1.20.3: Fix the build of sdl-perl
* Use the cpan class to do all the work instead of manually trying to do
  what cpan.bbclass does.
* Add some sed to stop it searching the system paths for SDL headers and to
  work around it expecting smpeg.h to be in an smpeg subdirectory.
* Update the homepage URL.
NOTE: The source for this version is no longer available, but it is the
version shipped by most distro's. I just grabbed the source from an
"apt-get source sdl-perl".
Fixes #413.