gwibber 3.1.6 does not build without gtkspell

While trying to build the upcoming gwibber for gentoo, I wanted to avoid patching gtkspell so I disabled gtkspell in the ./configure call (--disable-gtkspell). Configure runs through but the build fails:

  CCLD libgwibber.la
LD_LIBRARY_PATH=.libs \
/usr/bin/g-ir-compiler \
 --includedir=. \
 --includedir=. \
  Gwibber-0.2.gir -o Gwibber-0.2.typelib
Gwibber-0.2.gir:65:97: warning: element annotation from state 15 is unknown, ignoring
Gwibber-0.2.gir:66:65: warning: element annotation from state 15 is unknown, ignoring
Gwibber-0.2.gir:70:104: warning: element annotation from state 15 is unknown, ignoring
Gwibber-0.2.gir:71:88: warning: element annotation from state 15 is unknown, ignoring
Gwibber-0.2.gir:849:50: warning: element annotation from state 9 is unknown, ignoring
make[2]: Leaving directory `/home/tante/Downloads/gwibber-3.1.6/libgwibber'
Making all in libgwibber-gtk
make[2]: Entering directory `/home/tante/Downloads/gwibber-3.1.6/libgwibber-gtk'
  CC libgwibber_gtk_la-action-box.lo
  CC libgwibber_gtk_la-entry.lo
entry.c:33:31: fatal error: gtkspell/gtkspell.h: No such file or directory
compilation terminated.
make[2]: *** [libgwibber_gtk_la-entry.lo] Error 1
make[2]: Leaving directory `/home/tante/Downloads/gwibber-3.1.6/libgwibber-gtk'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/tante/Downloads/gwibber-3.1.6'
make: *** [all] Error 2