Hello Jeroen
I tried to google this a bit, one thing I found was http://sourceware.org/bugzilla/show_bug.cgi?id=5210, but I fail to (easily) find a reliable gcc reference. Perhaps we should file a bug? -Werror doesn't work with /dev/null when -fstack-usage is not supported...
I might be a bug but I cannot say with confident. Anyway, we need to do something with the current gcc behavior.
I cannot comment on your concern regarding Kbuild, since I am unaware of its internals. I just suggested above since it would be a cleaner approach (if it worked...).
I posted version2 : config.mk: disable -fstack-usage for blackfin and m68k and version3 : config.mk: fix -fstack-usage support test
I think the latter is close to your idea and I believe it's better.
Best Regards Masahiro Yamada