Include stdlib.h to fix:
main.c:69:19: error: implicit declaration of function 'atoi' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

Include unistd.h to fix:
main.c:65:17: error: implicit declaration of function 'getopt' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

Include tokens.h to fix:
main.c:48:5: error: implicit declaration of function 'yyparse' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

Specify return value of main to fix:
main.c:58:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]

Delete declaration of unused function getenv to fix:
main.c:66:28: warning: this function declaration is not a prototype [-Wstrict-prototypes]
--- main.c.orig	2015-11-21 17:42:11.000000000 -0600
+++ main.c	2023-01-12 21:07:44.000000000 -0600
@@ -16,9 +16,12 @@
 *****************************************************************************/
 /*LINTLIBRARY*/
 #include <stdio.h>
+#include <stdlib.h>
 #include <ctype.h>
 #include <string.h>
+#include <unistd.h>
 #include "cupl.h"
+#include "tokens.h"
 
 extern FILE *yyin;		/* the program text file descriptor */
 extern int yylineno;		/* the current source line count */
@@ -53,13 +56,13 @@
     return(0);
 }
 
+int
 main(argc, argv)
 int	argc;
 char	*argv[];
 {
     extern int	    optind;		/* getopt() sets this */
     extern char	    *optarg;		/* and this */
-    extern char	    *getenv();
     int	c;
 
     while ((c = getopt(argc, argv, "f:v:w:")) != EOF)