Building netbsd-8 and netbsd-9 with modern gcc

[David Brownlee <abs%absd.org@localhost>]

More recent gcc versions (including the gcc-10 in netbsd-10) default
to -fno-common, which breaks building netbsd-8 and netbsd-9.

This is an... unwelcoming initial experience for anyone downloading
the latest netbsd release and trying to build it. It breaks quite
quickly - when building nbmake (IIRC).

A workaround is relatively simple - pass "-V HOST_CFLAGS=-fcommon" to
build.sh, but does not appear to be directly documented anywhere.

What would be the best way to fix this - adjust tools/make/configure
so if "ac_compiler_gnu = yes" -fcommon would be added to HOST_CFLAGS?

David