Fixes
../src/time/ISO8601.cxx:67:24: error: use of undeclared identifier 'strtoul'
        unsigned long value = strtoul(s, &endptr, 10);
                              ^
../src/time/ISO8601.cxx:77:14: error: use of undeclared identifier 'strtoul'
                        minutes = strtoul(s, &endptr, 10);
                                  ^

on NetBSD with clang 9.0.0.