Avoid implicit function declarations; they are not available in C99.
No need to upstream this.  This shell code is created by autoconf,
and current autoconf no longer uses implicit declarations.

diff --git a/configure b/configure
index 53e720157484d948..255eaea6bb2200c7 100755
--- a/configure
+++ b/configure
@@ -2795,8 +2795,8 @@ main ()
   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$ac_exeext
@@ -4383,6 +4383,7 @@ _ACEOF
 cat confdefs.h >>conftest.$ac_ext
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
+#include <string.h>
 
 int
 main ()
@@ -4391,7 +4392,7 @@ main ()
   /* Some versions of memcmp are not 8-bit clean.  */
   char c0 = 0x40, c1 = 0x80, c2 = 0x81;
   if (memcmp(&c0, &c2, 1) >= 0 || memcmp(&c1, &c2, 1) >= 0)
-    exit (1);
+    return 1;
 
   /* The Next x86 OpenStep bug shows up only when comparing 16 bytes
      or more and with at least one buffer not starting on a 4-byte boundary.
@@ -4407,9 +4408,9 @@ main ()
         strcpy (a, "--------01111111");
         strcpy (b, "--------10000000");
         if (memcmp (a, b, 16) >= 0)
-          exit (1);
+          return 1;
       }
-    exit (0);
+    return 0;
   }
 
   ;