[Build] Add autogen.sh script to call `autoreconf -i`

Even when using autoreconf an autogen.sh script is usually expected, as
it save users from needing to know which flags to pass to autoreconf
(ie. '-i').

It is also usually responsible of launching autogen from the right
directory, calling utilities like intltoolize and gtkdocize, checking
out git submodules and running ./configure if $NOCONFIGURE is not set.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>