Update to 2.16.4 and switch to guile20

Add TEST_TARGET.  Some tests fail, but the calendar example runs.

Upstream changes:

Guile-Gnome-Platform 2.16.4 - David Pirotte, 2015-12-08
=======================================================

This release fixes a serious bug, leading to a runtime error for
Guile-Gnome built against Gcc >= 5.0.

Glib
----

* glib/gnome/gobject/gclosure.c: gcc_please_look_the_other_way must be
  declared volatile.

guile-gnome-platform 2.16.3 - David Pirotte, 2015-07-30
=======================================================

The primary objectives, working towards this release, were (1) to
remove all code specific to the support of guile versions prior to
guile 2, (2) getting rid of all guile related deprecated messages
[that second part of the mission almost completed, there is still one
corba related deprecated warning message, help to resolve that last
one is welcome!], and (3) wrap gdk_pixbuf_get_pixels required by
guile-clutter 1.12.

All modules
-----------

* Guile-Gnome now requires Guile 2.0, allows Guile 2.2
* Requires G-Wrap >= 1.9.15
* Requires Guile-Cairo >= 1.10

    Note that at the time we write, Guile-Cairo 1.10 has not been
    released yey, it will hopefully soon be the case.  In the mean
    time, the required bleeding edge Guile-Cairo stable version must
    be obtained from the source, see:

       http://www.nongnu.org/guile-cairo/dev/

* toplevel calls moved into an eval-when conditions exp...
* scm-error incorrect arg list, various calls, all fixed.
* all [but 1 Corba related] Guile deprecated calls removed
* remove use of some undocumented GOOPS internals
* adapt to GOOPS changes in Guile 2.2
* make check and make distcheck both have been fixed to work with
  recent automake versions [thanks to Ludovic Court辿s for his help wrt
  to make check]

Corba
-----

* fixing allocate-instance for <CORBA:Object>
* gnome-corba-error, gruntime-error, don't call save-stack
  [deprecated]

Glib
----

* g_get_current_time now [temporarily] ignored, it was not properly
  wrapped anyway.
* <glib-wrapset> missing some basic pointer types: guint8*, guchar*,
  gunichar*, gunichar2* added and wrapped as opaque pointers.
* scm_c_gparam_construct 4th argument type definition fixed
* use &gw-bad-typespec, not &gw:bad-typespec, fixed.

Gdk-Pixbuf
----------

* gdk_pixbuf_get_pixels removed from the ignore list, it returns a
  guchar*, just added, no specific wrapper needed.

Gtk
---

* gtk_recent_info_get_application_info should [temporarily] be
  ignored, it needs a special wrapper, to be done still.


(gdt)