libgtk2: break the dependency of host-libgtk2 on host-cairo

Making a full build of libgtk2 for the host is a pain, since it
requires host-cairo, which it turns requires a full X.org stack to be
compiled for the host.

Moreover, building libgtk2 for the host completely is useless: we only
need gdk-pixbuf-csource and gtk-update-icon-cache.

In this patch, we add a new
host-libgtk2-2.12.12-reduce-dependencies.patch, that is only applied
to libgtk2 when being built for the host. This patch removes a lot of
dependencies in configure.in, which allows to:

 * run make and make install in gdk-pixbuf/, which compiles and
   installs the gdk-pixbuf library and the gdk-pixbuf-csource utility

 * run make gtk-update-icon-cache in gtk/, which compiles the
   gtk-update-icon-cache utility, which is then installed manually

The compilation and installation steps of libgtk2 for the host are
therefore overriden, so as to not use the default behaviour, which
would attempt to build the full gtk library.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>