gtk-doc: add gtk-doc.m4 to satisfy aclocal

When packages using gtk-doc are autoreconfigured, aclocal complains
because it cannot find the macros defined in gtk-doc.m4. We could
compile the gtk-doc package for the host, but it depends on
gnome-doc-utils, which depends on libxml2, libxslt, and other packages
as well.

Since we don't care about the documentation, all is needed is in fact
the gtk-doc.m4, so that the configure script can be generated, and we
can use the --disable-gtk-doc to not generate the documentation.

To solve this, we include a gtk-doc.m4 file in package/automake/, and
it gets installed in $(STAGING_DIR)/usr/share/aclocal/ during the
installation of the host automake (used for autoreconfiguration of
packages).

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