configure.ac currently lists the minimum required version as
GTK_MIN_VERSION=3.16

However, if gnome-dictionary is built against a version < 3.20, it fails to build:

gdict-defbox.c: In function 'defbox_visibility_notify_cb':
gdict-defbox.c:1523:3: error: unknown type name 'GdkSeat'
   GdkSeat *seat;
   ^
gdict-defbox.c:1529:10: warning: implicit declaration of function 'gdk_display_get_default_seat' [-Wimplicit-function-declaration]
   seat = gdk_display_get_default_seat (display);
          ^
gdict-defbox.c:1529:8: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
   seat = gdk_display_get_default_seat (display);
        ^
gdict-defbox.c:1530:13: warning: implicit declaration of function 'gdk_seat_get_pointer' [-Wimplicit-function-declaration]
   pointer = gdk_seat_get_pointer (seat);
             ^
gdict-defbox.c:1530:11: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
   pointer = gdk_seat_get_pointer (seat);
           ^
Makefile:702: recipe for target 'libgdict_1_0_la-gdict-defbox.lo' failed
make[4]: *** [libgdict_1_0_la-gdict-defbox.lo] Error 1
make[4]: Leaving directory '/«PKGBUILDDIR»/libgdict'
Makefile:535: recipe for target 'all' failed
make[3]: *** [all] Error 2
make[3]: Leaving directory '/«PKGBUILDDIR»/libgdict'
Makefile:484: recipe for target 'all-recursive' failed
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory '/«PKGBUILDDIR»'
Makefile:416: recipe for target 'all' failed
make[1]: *** [all] Error 2
make[1]: Leaving directory '/«PKGBUILDDIR»'
/usr/share/cdbs/1/class/makefile.mk:47: recipe for target 'debian/stamp-makefile-build' failed
make: *** [debian/stamp-makefile-build] Error 2
dpkg-buildpackage: error: debian/rules build-arch gave error exit status 2


Full build log is at
https://buildd.debian.org/status/fetch.php?pkg=gnome-dictionary&arch=mipsel&ver=3.20.0-1&stamp=1461766385


It seems gdk_seat_get_pointer() [1] was introduced in 3.20 
and so was gdk-display-get-default-seat() [2].

Please bump the minimum required version accordingly.

[1] https://developer.gnome.org/gdk3/stable/GdkSeat.html#gdk-seat-get-pointer
[2] https://developer.gnome.org/gdk3/stable/GdkDisplay.html#gdk-display-get-default-seat