Comment 57 for bug 44728

Copying files out of the Pidgin tarball is not cleaner. It requires that on every upgrade, someone copy a tree of files. Additionally, as you can see, internal stuff uses internal.h, which doesn't belong in libpurple-dev.

If I was to want to split this (which I think is a terrible idea compared to just building pidgin with SILC support in main), I would take the pristine upstream tarball and use that as the base of a pidgin-silc source package. You should configure with --disable-gtk --disable-consoleui. This should build just libpurple. (I haven't tested it, but if it doesn't, we can fix that upstream.) Then, instead of calling "make install" from your package's install target, you'd install just the single .so of the SILC prpl from debian/rules. Also, you'll want the pidgin-silc binary package to depend on libpurple >= ${source:Version}. This is more specific than you probably need (as you really only care about the major and minor, not the micro), but it's easier than rigging up some sort of script to create a substvar that's not really needed in practice.

This way, you'd get the results you want without needing to complicate future package maintenance. You'd just need to grab a new Pidgin tarball and rebuild the package.