I have a pre-ANSI compiler, without stdarg.h What can I do?

There's an older header, <varargs.h>, which offers about the same functionality.
Here is the vstrcat function, rewritten to use <varargs.h>:
#include <stdio.h>
#include <varargs.h>
#include <string.h>

extern char *malloc();

char *vstrcat(va_alist)
va_dcl /* no semicolon */
{
int len = 0;
char *retbuf;
va_list argp;
char *p;

va_start(argp);

while((p = va_arg(argp, char *)) != NULL) /* includes first */
len += strlen(p);

va_end(argp);

retbuf = malloc(len + 1); /* +1 for trailing \0 */

if(retbuf == NULL)
return NULL; /* error */

retbuf[0] = '\0';

va_start(argp); /* restart for second scan */

while((p = va_arg(argp, char *)) != NULL) /* includes first */
strcat(retbuf, p);

va_end(argp);

return retbuf;
}

(Note that there is no semicolon after va_dcl. Note that in this case, no special treatment for the first argument is necessary.) You may also have to declare the string functions by hand rather than using <string.h>.
If you can manage to find a system with vfprintf but without <stdarg.h>, here is a version of the error function using <varargs.h>:>

#include <stdio.h>
#include <varargs.h>

void error(va_alist)>
va_dcl /* no semicolon */>
{>
char *fmt;>
va_list argp;>
fprintf(stderr, "error: ");>
va_start(argp);>
fmt = va_arg(argp, char *);>
vfprintf(stderr, fmt, argp);>
va_end(argp);>
fprintf(stderr, "\n");>
}>
>
(Note that in contrast to <stdarg.h>, under < all arguments are variable, so the fmt argument must also be picked up via va_arg.)