How to parse SimpleTerminal code?

Question

C++ / C#

I started to disassemble the SimpleTerminal code and a few misunderstandings arose.
1) Why is the file initially called config.def.h, and there are lines in the Makefile

config.h:
  cp config.def.h config.h

What for?
2) The arg.h file has a define that parses the arguments

/* use main(int argc, char *argv[]) */
#define ARGBEGIN	for (argv0 = *argv, argv++, argc--;\
          argv[0] && argv[0][0] == '-'\
          && argv[0][1];\
          argc--, argv++) {\
        char argc_;\
        char **argv_;\
        int brk_;\
        if (argv[0][1] == '-' && argv[0][2] == '\0') {\
          argv++;\
          argc--;\
          break;\
        }\
        int i_;\
        for (i_ = 1, brk_ = 0, argv_ = argv;\
            argv[0][i_] && !brk_;\
            i_++) {\
          if (argv_ != argv)\
            break;\
          argc_ = argv[0][i_];\
          switch (argc_)

#define ARGEND			}\
      }

// В x.c используется так 
int main(int argc, char *argv[])
{ ...
  ARGBEGIN {
  case 'a':
    allowaltscreen = 0;
    break;
  case 'c':
    opt_class = EARGF(usage());
    break;
  case 'e':
    if (argc > 0)
      --argc, ++argv;
    goto run;
  case 'f':
    opt_font = EARGF(usage());
    break;
  case 'g':
    xw.gm = XParseGeometry(EARGF(usage()),
        &xw.l, &xw.t, &cols, &rows);
    break;
  case 'i':
    xw.isfixed = 1;
    break;
  case 'o':
    opt_io = EARGF(usage());
    break;
  case 'l':
    opt_line = EARGF(usage());
    break;
  case 'n':
    opt_name = EARGF(usage());
    break;
  case 't':
  case 'T':
    opt_title = EARGF(usage());
    break;
  case 'w':
    opt_embed = EARGF(usage());
    break;
  case 'v':
    die("%s " VERSION "\n", argv0);
    break;
  default:
    usage();
  } ARGEND;
....
}

What? What for?
Why argv[0] && argv[0][0] == '-'&& argv[0][1];check the first character (of the file?) for '-'?

Answer 1

1) Perhaps this happens with some compilation options? Type a certain default file is taken, and in other cases some other one.
2) Because the first thing there is argv++

Answer 2

1) Obviously, if there is no config.h file (the goal is not achieved), a default config file is created.
2) It is not the first character of the file that is checked, but the correctness of the format of the passed arguments.