build: downgraded intltool dependency to 0.35.0

My web server is based on CentOS 5 and has intltool 0.35.0 installed,
and ADG do not rely on a specific i18n feature.
adg: make cairo-gobject optional

cairo-gobject has been added to cairo only recently (2010), so including
it only optionally. In case it does not exist, use the yet existing
fallbacks (or provide the missing ones if there will be).
build: make --internal of glib-genmarshal optional

glib-genmarshal --internal has been introduced in glib 2.13.0. Check the
GLib version and eventually strip this flag from the makefiles.
adg: provided cairo-gobject fallbacks

Consider cairo-gobject entirely optional and implement fallbacks for
every GObject wrapper provided by it, i.e. only for cairo_pattern_t.
build: stripped --internal arg from glib-genmarshal

On CentOS 5 (a fairly old GNU/Linux dist), the --internal flag of
glib-genmarshal generates the following error while linking:

/usr/bin/ld: .libs/libadg_1_la-adg-container.o: relocation R_X86_64_PC32
against `adg_marshal_VOID__OBJECT' can not be used when making a shared
object; recompile with -fPIC
build: depends on cairo-gobject if introspection is enabled

If introspection is used, GObject support in cairo must be enabled
because gobject-introspection package relies on the presence of this
wrapper library. That support is present since cairo 1.10.0.
doc: updated NEWS.xml
build: bumped version to 0.7.3