Cause autogen.sh to fail if autoreconf fails

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #7683