how to handle: a package needs another package with a particular option

I'm trying to put together a package for the S/MIME plug-in for
Claws-mail (currently sylpheed-claws in pkgsrc; I'll take care of that
renaming as soon as the current freeze is over).  The problem is that
the plug-in requires gpgsm, which is installed as part of gnupg2 -- but
only if the 'gpgsm' option is defined for it.  How do I handle that?
(There are comments in the latter's Makefile indicating that perhaps
its modularization is wrong, but I'm not going to tackle that,
especially since smime-plugin needs another piece of gnupg2 as well.)


                --Steve Bellovin, http://www.cs.columbia.edu/~smb