Hi,
I guess newer Ubuntu versions don't come with libdevhelp-2.0 but instead only libdevhelp-3.0 (for GTK+ 3). The Devhelp plugin should be able to work fine with libdevhelp-3.0, but I cannot test it and it won't work anyway because Geany does not support GTK+ 3.
I was just looking into putting the source code for libdevhelp 2.32.0 (for GTK+ 2.0) statically into the Devhelp plugin's source tree and making Geany Plugins' build system compile it straight into the Devhelp plugin. It's seems like it's going to be a pain and I'm not totally sure I even want to do it.
So anyway, even if I go this route, I probably won't be able to get it working and tested in time for packaging for Ubuntu 11.10+ of Geany Plugins 0.21 so don't bother trying to get it working for these packages. It also won't work on Debian testing/unstable and probably not on Win32 either.
If anyone wants to use the Devhelp plugin with these newer distros, they'll have to install an older libdevhelp-2.0 package or compile Devhelp <= 2.32.0 from source and then obviously build the Devhelp plugin from Geany-Plugins' source.
Cheers, Matthew Brush