Avoid implicit function declarations in the configure script.  This
helps to ensure compatibility with future compilers, which are likely
to reject them by default.

In the unlikely event of another upstream release, current autoconf no
longer produces these script fragments, so the issue should go away on
its own.

diff --git a/configure b/configure
index e49342eb47e20ca1..9a331621d4059d1c 100755
--- a/configure
+++ b/configure
@@ -2124,7 +2124,7 @@ if { (eval echo "$as_me:$LINENO: \"$ac_compile\"") >&5
   echo "$as_me:$LINENO: \$? = $ac_status" >&5
   (exit $ac_status); }; }; then
   for ac_declaration in \
-   '' \
+   '#include <stdlib.h>' \
    'extern "C" void std::exit (int) throw (); using std::exit;' \
    'extern "C" void std::exit (int); using std::exit;' \
    'extern "C" void exit (int) throw ();' \
@@ -3221,8 +3221,8 @@ main ()
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext
@@ -4697,6 +4697,9 @@ cat >>conftest.$ac_ext <<_ACEOF
 #include <sys/time.h>
 #include <sys/resource.h>
 #include <stdio.h>
+#include <unistd.h>
+#include <sys/wait.h>
+#include <stdlib.h>
 /* HP-UX has wait3 but does not fill in rusage at all.  */
 int
 main ()