Add check stage for documentation submodules.

This adds an extra check when XDT_AUTOGEN_CHECK_DOCS
is defined. This variable points to the directory that
should contain the xfce4-docs submodule.

If no submodule clone if found and autogen.sh is running
from within a git clone, it will automatically initialize
the submodule for the user, if not it will show an error
and abort xdt-autogen, since configure will abort too if
we continue because of missing makefile.am files.