ade: Warn on unknown arguments without -- pseudo-argument

Picking out the known arguments and leaving unknown arguments is
dangerous, because typos and prefix-matching can have unintended
results. To discourage this, warn if any unknown arguments were
found. These are all considered correct, and are equivalent
(except for order):

    ade configure --native -- VARIABLE=1 --with-option
    ade configure --native VARIABLE=1 --with-option
    ade configure --native -- --with-option VARIABLE=1

but this will now provoke a warning for --with-option:

    ade configure --native --with-option VARIABLE=1

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: André Magalhães <andre.magalhaes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D6797