AUTOMAKE_OPTIONS= -Werror

If I want automake warnings to be fatal throughout the entire tree, is it
better to:

- run automake (or autoreconf) with -Werror

- Put -Werror in the top-level Makefile.am's AUTOMAKE_OPTIONS

- Put -Werror in every Makefile.am's AUTOMAKE_OPTIONS

I am thinking about the case where somebody gets a changed subdir
Makefile.am (from an update from the source code management system, for
example) and it gets rebuilt automatically.

I suspect the answer is that it has to go everywhere, but I wanted to ask to
be sure.

H