Fix:

error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]

error: implicit declaration of function 'fork' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

error: implicit declaration of function 'getpid' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

error: implicit declaration of function 'getpgrp' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

error: implicit declaration of function 'setpgrp' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

error: implicit declaration of function 'wait' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

error: implicitly declaring library function 'strncmp' with type 'int (const char *, const char *, unsigned long)' [-Werror,-Wimplicit-function-declaration]

error: implicitly declaring library function 'strcmp' with type 'int (const char *, const char *)' [-Werror,-Wimplicit-function-declaration]

error: implicitly declaring library function 'toupper' with type 'int (int)' [-Werror,-Wimplicit-function-declaration]
--- src/configure.orig	2020-12-07 18:25:30.000000000 -0600
+++ src/configure	2020-12-07 18:36:45.000000000 -0600
@@ -1186,8 +1186,8 @@
 #define TOUPPER(c) (ISLOWER(c) ? 'A' + ((c) - 'a') : (c))
 #define XOR(e, f) (((e) && !(f)) || (!(e) && (f)))
 int main () { int i; for (i = 0; i < 256; i++)
-if (XOR (islower (i), ISLOWER (i)) || toupper (i) != TOUPPER (i)) exit(2);
-exit (0); }
+if (XOR (islower (i), ISLOWER (i)) || toupper (i) != TOUPPER (i)) return 2;
+return 0; }
 
 EOF
 if { (eval echo configure:1337: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -1372,6 +1372,9 @@
  */
 #include <stdio.h>
 #include <sys/types.h>
+#include <sys/wait.h>
+#include <stdlib.h>
+#include <unistd.h>
 
 int     pid;
 int     pg1, pg2, pg3, pg4;
--- src/guess.c.orig	2003-11-12 21:08:19.000000000 -0600
+++ src/guess.c	2020-12-07 18:55:13.000000000 -0600
@@ -21,6 +21,7 @@
  */
 
 #include <stdio.h>
+#include <string.h>
 
 #include <import.h>
 #include <decode.h>
--- src/guesslocale.c.orig	2004-01-05 02:41:22.000000000 -0600
+++ src/guesslocale.c	2020-12-07 18:55:41.000000000 -0600
@@ -22,6 +22,7 @@
 
 #ifdef HAVE_SETLOCALE
 
+#include <ctype.h>
 #include <stdlib.h>
 #include <string.h>
 #include <locale.h>
--- src/configure.orig	2024-10-17 16:25:33
+++ src/configure	2024-10-17 16:30:17
@@ -675,7 +675,7 @@
 #line 676 "configure"
 #include "confdefs.h"
 
-main(){return(0);}
+int main(void){return(0);}
 EOF
 if { (eval echo configure:681: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext}; then
   ac_cv_prog_cc_works=yes
@@ -1380,7 +1380,7 @@
 int     pg1, pg2, pg3, pg4;
 int     ng, np, s, child;
 
-main()
+int main(void)
 {
         pid = getpid();
         pg1 = getpgrp(0);
@@ -1416,6 +1416,7 @@
                 wait(&s);
                 exit(s>>8);
         }
+        return 0;
 }
 
 EOF