During the autoconf and autoreconf process, we pass a -Wcross to the autoconf and that is tranlated to --warnings=cross to automake.
will look into it while upgrading automake.
A good example is: libfm-0.1.17-r3/temp/log.do_configure autoreconf: running: automake --foreign --add-missing --copy --force-missing --warnings=cross automake: unknown warning category `cross'
I think your recent changes are close, it might be upstreamable
Adapted autoconf to pass --warning=cross to automake only if supported.