gtk+3: add gtk+-native dependency

* it's needed to provide gtk-update-icon-cache
  --enable-gtk2-dependency implies external gtk-update-icon-cache
  AM_CONDITIONAL(USE_EXTERNAL_ICON_CACHE, [test x = xyes || test x = xyes])
  unfortunately result of AC_PATH_PROG(GTK_UPDATE_ICON_CACHE, gtk-update-icon-cache, no)
  is not checked for no value before being used resulting in
  no --quiet --force --ignore-theme-index               \
          --source builtin_icons stock-icons > gtkbuiltincache.h.tmp &&        \
       mv gtkbuiltincache.h.tmp gtkbuiltincache.h
  /bin/sh: no: command not found

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>