Fix:

error: call to undeclared library function 'exit' with type 'void (int) __attribute__((noreturn))'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
--- configure.orig	2021-01-12 12:41:17.000000000 -0600
+++ configure	2021-01-12 12:46:17.000000000 -0600
@@ -4952,8 +4952,8 @@
   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
@@ -9804,7 +9804,7 @@
   else
     puts (dlerror ());
 
-    exit (status);
+  return status;
 }
 EOF
   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5
@@ -9885,10 +9885,6 @@
 #  endif
 #endif
 
-#ifdef __cplusplus
-extern "C" void exit (int);
-#endif
-
 void fnord() { int i=42;}
 int main ()
 {
@@ -9904,7 +9900,7 @@
   else
     puts (dlerror ());
 
-    exit (status);
+  return status;
 }
 EOF
   if { (eval echo "$as_me:$LINENO: \"$ac_link\"") >&5