autogen.sh: use exec instead of waiting for configure to finish

Syncs the invocation of configure with the one from the server.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>