It's annoying for distributors to build with -Werror, since it means
that compiler upgrades can break the build. Let's let them disable it,
but keep it enabled by default.
We now don't break the strict aliasing rules.
Also, having a compiler flag that disables a warning among the flags that are
meant to add extra sanity checking is not correct either:
--enable-extra-warnings=no would generate a bad aliasin warning while
the --enable-extra-warnings=yes would not.
GValueArray is deprecated since GLib 2.31.14, but we need to use it for
dbus-glib based code.
We should re-enable deprecation warnings once we switch to GDBus.