Eh...

  // This code is licensed under CC0.
  // A copy of the license can be obtained at https://creativecommons.org/publicdomain/zero/1.0/
  // May you forever catenate in peace.
  #define strcatm(...) strcat_multi(__VA_ARGS__, NULL);
  char* strcat_multi(char *dest, ...) {
    va_list srcs;
    char *dest_end;
    const char *src;
    size_t src_sz;
    va_start(srcs, dest);
    for (dest_end = dest; *dest_end; dest_end++);
    for (src = va_arg(srcs, const char *); src; src = va_arg(srcs, const char *)) {
      src_sz = strlen(src);
      memmove(dest_end, src, src_sz);
      dest_end += src_sz;
      *dest_end = '\0';
    }
    va_end(srcs);
    return dest_end;
  }

There. Now go and catenate, children.