The building process should follow the usual
 ./configure
 make
 make install
procedure. The plugin gets installed into
/usr/local/lib/claws-mail/plugins/

If the building process fails, consult `perlembed' from the Perl
documentation, section `Compiling your C program' (yes, that is
the one starting with "If you have trouble compiling the scripts
in this documentation, you're not alone."). If you successfully
use the plugin on a non - i[3-6]86 platform, please drop me a
note.