build: enable all warnings as fatal in our own build system

build: enable all warnings as fatal in our own build system

Automake should of course be able to bootstrap itself in a
warning-free manner w.r.t. the Autotools.  So make any failure
to do so fatal.  Not doing so caused the regression fixed by
previous commit 'v1.13.1-22-ga790fae' to go unnoticed.

* configure.ac (AM_INIT_AUTOMAKE): Add '-Werror' and '-Wall'.
* bootstrap.sh: Pass the '-Wall -Werror' options to aclocal,
automake and autoconf invocations.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>