Ok, I'm writing this while looking at 2.3.14 sources, but as 2.3.4 ebuilds already were that way, I suspect that this considers both later ebuilds that are in the tree (2.3.8 and 2.3.13). Those two set dependency to gtkhtml? ( =gnome-extra/gtkhtml-2* ) although balsa supports (and prefers) gtkhtml-3 (as stated by configure.in).
This is fixed in balsa 2.3.16