build: don't use -Werror in AM_INIT_AUTOMAKE

Doing so prevents bootstrapping with bleeding-edge autotools,
because of harmless deprecation warnings (that are not planned
to become hard errors for at least a few years to come).  And
unfortunately, options in AM_INIT_AUTOMAKE take precedence over
those given on the command line (this is a long-time wart of
automake).

* configure.ac (AM_INIT_AUTOMAKE): Drop '-Werror' option.

Copyright-paperwork-exempt: yes
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>