Only use Gtk::Window::set_default_icon_name method when available (#695279)

GParted fails to compile on RHEL/CentOS 5.9 because it doesn't provide
the Gtk::Window::set_default_icon_name() method.  The "gtkmm GTK::Window
Class Reference" document says that set_default_icon_name() is available
in gtkmm >= 2.6, however it is not available in RHEL/CentOS 5.9 with
gtkmm 2.10.

Add an autoconf compile and link check for the set_default_icon_name()
method and make GParted only use the method when available.

Bug #695279 - GParted doesn't compile on RHEL / CentOS 5.9