When do I need autogen and configure?

[Thorsten Meyer]

Hi,

new patches have been commited to savannah at an amazing rate lately and I
certainly hope that it stays like this :-).
However, with my rather old computer, it gets increasingly difficult to keep my
mercurial octave up to date, because building octave takes so long (even with
ccache).

My question in this context:
How can I figure out, if I really need to rerun autogen or configure to build
octave? Could there be a way to let the Makefiles figure it out for themselves?

thanks

Thorsten