Add support for gtkmm-3.0 and prefer it over gtkmm-2.* by default.

Pass --disable-gtk3 to configure to build against gtk2

configure.ac

@@ -39,7 +39,26 @@ AC_PROG_LN_S
 AC_TYPE_SIGNAL
 AC_HEADER_STDC
 
-PKG_CHECK_MODULES(GUILIBS, [ gtkmm-2.4 >= 2.16 sigc++-2.0 libcanberra-gtk >= 0.16 ])
+AC_ARG_ENABLE(gtk3,
+   [AS_HELP_STRING([--disable-gtk3],
+      [Whether to build or not against gtk3 [default=yes]])],
+   [enable_gtk3=$enableval],
+   [enable_gtk3="yes"])
+
+if test "x$enable_gtk3" = "xyes"; then
+   gtkmm_ver="3.0"
+   gtkmm_min_ver="2.99"
+   canberra_ver="3"
+   AC_DEFINE([HAVE_GTK3], [1], [Define if we are building against gtk3])
+   echo "***"
+   echo "*** Attempting build in GTK3 mode. Use --disable-gtk3 to build for GTK2"
+   echo "***"
+else
+   gtkmm_ver="2.4"
+   gtkmm_min_ver="2.16"
+   canberra_ver=""
+fi
+PKG_CHECK_MODULES(GUILIBS, [ gtkmm-$gtkmm_ver >= $gtkmm_min_ver sigc++-2.0 libcanberra-gtk$canberra_ver >= 0.16 ])
 AC_SUBST(GUILIBS_CFLAGS)
 AC_SUBST(GUILIBS_LIBS)
 

src/mainwindow.cc

@@ -152,7 +152,11 @@ MainWindow* MainWindow::create() {
 void MainWindow::on_realize() {
     Gtk::Window::on_realize();
 
+#ifdef HAVE_GTK3
+    get_window()->set_cursor(Gdk::Cursor::create(Gdk::WATCH));
+#else
     get_window()->set_cursor(Gdk::Cursor(Gdk::WATCH));
+#endif /* HAVE_GTK3 */
 }
 
 MainWindow::~MainWindow() {