Make GTK3 default.

configure.ac

@@ -46,12 +46,12 @@ IT_PROG_INTLTOOL([0.35.0])
 
 AC_MSG_CHECKING([which gtk+ version to compile against])
 AC_ARG_WITH([gtk],
-  [AS_HELP_STRING([--with-gtk=2.0|3.0],[which gtk+ version to compile against (default: 2.0)])],
+  [AS_HELP_STRING([--with-gtk=2.0|3.0],[which gtk+ version to compile against (default: 3.0)])],
   [case "$with_gtk" in
      2.0|3.0) ;;
      *) AC_MSG_ERROR([invalid gtk version specified]) ;;
    esac],
-  [with_gtk=2.0])
+  [with_gtk=3.0])
 AC_MSG_RESULT([$with_gtk])
 
 case "$with_gtk" in