drawingarea/image and menus_and_toolbars examples: Use resource files

* configure.ac: Add check for C compiler and glib-compile-resources.
* examples/.gitignore: Ignore resources.c.
* examples/Makefile.am: Add make rules for the resource files.
* examples/book/drawingarea/image/myarea.cc: Use Gdk::Pixbuf::
create_from_resource() instead of create_from_file().
* examples/book/drawingarea/image/image.gresource.xml: New file.
* examples/book/menus_and_toolbars/examplewindow.cc: Use Gtk::Builder::
add_from_resource().
* examples/book/menus_and_toolbars/toolbar.glade:
* examples/book/menus_and_toolbars/toolbar.gresource.xml: New files.
Add the "Gio::Resource and glib-compile-resources" section

* docs/tutorial/C/index-in.docbook: Add the "Gio::Resource and glib-compile-
resources" section. Mention that the drawingarea/image example uses a resource
file.