I managed to get it to work by installing various other GNOME devel packages that gtkhtml2-devel doesn't explicitly depend on, but needs anyway to be recognized by pkg-config. These are more apt to be installed by default on a GNOME system, but not necessarily by SuSE, which is why it has problems. So it seems to me that it might not actually be SuSE's fault that pkg-config doesn't find them.
pkg-config --libs libgtkhtml-3.0 gtkspell-2.0 librsvg-2.0
-pthread -Wl,--export-dynamic -L/opt/gnome/lib -L/usr/X11R6/lib -lgtkhtml-3.0 -lgtkhtml-a11y-3.0 -lgnomeui-2 -lSM -lICE -lgnomeprintui-2-2 -lgal-2.0 -lbonoboui-2 -lgnome-2 -lbonobo-2 -lgconf-2 -lgnomevfs-2 -lbonobo-activation -lORBit-2 -llinc -lgthread-2.0 -lgnomecanvas-2 -lglade-2.0 -lgnomeprint-2-2 -lgtkspell -laspell -lgtk-x11-2.0 -lgdk-x11-2.0 -latk-1.0 -lpangoxft-1.0 -lpangox-1.0 -lrsvg-2 -lgdk_pixbuf-2.0 -lart_lgpl_2 -lpangoft2-1.0 -lgsf-1 -lpango-1.0 -lgmodule-2.0 -ldl -lgobject-2.0 -lxml2 -lpthread -lz -lm -lglib-2.0
...seems to indicate what other libs pkg-config needs to be happy. The packages that provide all the libraries mentioned are: atk-devel, bonobo-activation-devel, gconf2-devel, glib2-devel, gnome-vfs2-devel, gtkhtml2-devel, libart_lgpl-devel, libbonobo-devel, libbonoboui-devel, libgnomecanvas-devel, libgnome-devel, libgnomeprint-devel, libgnomeprintui-devel, libgnomeui-devel, libgsf-devel, librsvg-devel, libxml2-devel, orbit2-devel, and pango2-devel, along with whatever they depend on. Obviously this looks like a lot, but it may not all be necessary, but I have all these packages installed, and they're all referenced in the pkg-config output above. If you can help narrow down the list, great.
I've tried to build an RPM, but the build process keeps failing due to something with gdk-pixbuf, so the best I can do is recommend you try adding the aforementioned GNOME devel packages.
On the subject of HTML preview, it'd be nice to be able to have a "Preview" button right next to the Submit one. I always preview before I post, so it'd be very useful for me. Otherwise, 4.3 is fantastic.