build: bump gtk+ version requirement to 2.14
build: bump gtk+ version requirement to 2.14

Aravis uses gtk_widget_get_window, which appeared in gtk 2.14.

AnilRamachandran/aravis,AnilRamachandran/aravis,AravisProject/aravis,AravisProject/aravis,AnilRamachandran/aravis,AravisProject/aravis,AravisProject/aravis,AnilRamachandran/aravis,AravisProject/aravis,AnilRamachandran/aravis