Change to autoconf PKG_CHECK_EXISTS for set_default_icon_name() method (#762184)

Previously the autoconf check for Gtk::Window::set_default_icon_name()
method was a compile test because the documentation reported the method
was available in gtkmm from 2.6 [1], however it wasn't available on
RHEL / CentOS 5.x with gtkmm 2.10.

Then commit [2] added detection and enabling of C++11 compilation, but
after the above autoconf check.  So on Fedora 23 the compiler based
autoconf check for set_default_icon_name() method failed because C++11
compilation had not yet been enabled:

>   checking for Gtk::Window::set_default_icon_name method... no
    checking for gtk_show_uri function... yes
    checking for Gtk::MessageDialog::get_message_area() method... yes
>   checking for glibmm >= 2.45.40 which requires C++11 compilation... yes
>   checking whether g++ supports C++11 features by default... no
>   checking whether g++ supports C++11 features with -std=gnu++11... yes

The gtkmm source code reveals that set_default_icon_name() method was
only added in gtkmm 2.11.1 [3] so switch to a PKG_CHECK_EXISTS for this
version of gtkmm.

[1] gtkmm GTK::Window Class Reference
    https://developer.gnome.org/gtkmm/3.6/classGtk_1_1Window.html#a533d03e9b92d8ccd142ab3a44005cae4

[2] Enable C++11 compilation when using glibmm 2.45.40 and later (#756035)
    d6d7cb2b

[3] gtkmm NEWS file
    https://git.gnome.org/browse/gtkmm/tree/NEWS?h=gtkmm-2.14.0#n565

Bug 762184 - Autoconf check for C++11 comes after compile test for
             Gtk::Window::set_default_icon_name()