That sounds like fontconfig. It’s an open source library, remember - checking for documentation is hardly a conclusive way to determine its absence :S
Look for fontconfig.h and fontconfig.so (or whatever Solaris calls shared libraries ... I don’t have it installed yet).
I strongly suspect it’ll be present, given the Fc* manpages and the fact that Solaris 10 has GNOME, which tends to use fontconfig (and may require it, not sure).
You may need to tweak your PKG_CONFIG_PATH env var to include the directory where ’fontconfig.pc’ is if Scribus’s `configure’ script doesn’t find it the first time around.—Craig