string_concat reads the args with va_arg(ap, const unsigned char *),
and the NULL macro may have the wrong type (e.g. int).
Many places pass string literals of type char * to string_concat.
This is in principle also a violation, but I'm ignoring it for now
because if it becomes a problem with some C implementation, then so
will the use of unsigned char * with printf "%s", which is so
widespread in ELinks that I'm not going to try fixing it now.
The configure script no longer recognizes "CONFIG_UTF_8=yes" lines
in custom features.conf files. They will have to be changed to
"CONFIG_UTF8=yes". This incompatibility was deemed acceptable
because no released version of ELinks supports CONFIG_UTF_8.
The --enable-utf-8 option was not renamed.
Finally, we are able to identify the builds more accurately. Now we check
if commit-id (cogito tool) is available and pass the output to vernum.c
compilation which then exports it as a build_id string.
The first line of --version is now a bit long, we should probably move the
build date to a second indented line or something.
