Log Message:
Organise the Makefile and add a framework to allow building the plugins
found in sylpheed-claws distribution.  Note that this is _only_ meant for
included plugins.  The image-viewer and dillo-viewer packages are in the
pipe.

>From the sylpheed-claws package point of view, this is just a reordering
of the Makefile.  It breaks pkglint'ing, but it's not like there really is
a choice.