Posted today on comp.unix.programmer: -Werror is a gcc option turning all warnings into fatal errors. The m4 1.4.17 build system enables -Werror and wants to enable a warning called "mudflap". gcc 8.3.0 (Debian 10) doesn't support this warning anymore. Trying to enable a warning which isn't supported causes a warning. Hence, m4 1.4.17 can't be built with gcc 8.3.0 because the warning caused by trying to enable a warning which doesn't exist anymore is turned into a fatal error by -Werror.
