Fix:

error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
--- configure.orig	2007-02-19 00:32:39.000000000 -0600
+++ configure	2020-12-06 21:47:00.000000000 -0600
@@ -5023,8 +5023,8 @@
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext
@@ -14000,10 +14000,6 @@
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -14019,7 +14015,7 @@
   else
     puts (dlerror ());
 
-    exit (status);
+  return status;
 }
 EOF
   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
@@ -14100,10 +14096,6 @@
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -14119,7 +14111,7 @@
   else
     puts (dlerror ());
 
-    exit (status);
+  return status;
 }
 EOF
   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
