Configure: Place the argument checks at the beginning of execution

Configure: Place the argument checks at the beginning of execution

The autotool provides a simple mechanism which allows us to move ("divert")
the checks we do on the user arguments to the beginning of the script, yet
without needing to scatter the code.

This is good because we can raise errors very fast, user do not have to
wait until many other checks have passed before knowing he has to correct
his argument list; yet on our side we can keep related things together.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>