Switch to gtk+ 2.24 (fixed compilation issue on Ubuntu 16.04)

packaging/appimage/3rdparty/ext_gtk+/CMakeLists.txt

 ExternalProject_Add(
   ext_gtk+
   DOWNLOAD_DIR ${EXT_DOWNLOAD_DIR}
-  URL http://ftp.gnome.org/pub/GNOME/sources/gtk+/2.23/gtk+-2.23.90.tar.gz
-  URL_MD5 404c75655bc497994c13a1aa92c9949a
+  URL http://ftp.gnome.org/pub/GNOME/sources/gtk+/2.24/gtk+-2.24.32.tar.xz
+  URL_MD5 d5742aa42275203a499b59b4c382a784
 
   CONFIGURE_COMMAND <SOURCE_DIR>/configure --prefix=${EXT_INSTALL_DIR} ${AT_HOST} --disable-cups
   BUILD_COMMAND $(MAKE)