Fix C99 compatibility issues in the autoconf-generated code.  This
has already been fixed in autoconf, so there is no need to upstream
this change for sage.

diff --git a/configure b/configure
index b8060da2cecf8c49..4de2f86274c03abb 100755
--- a/configure
+++ b/configure
@@ -3000,7 +3000,7 @@ if { (eval echo "$as_me:$LINENO: \"$ac_compile\"") >&5
   echo "$as_me:$LINENO: \$? = $ac_status" >&5
   (exit $ac_status); }; }; then
   for ac_declaration in \
-   '' \
+   '#include <stdlib.h>' \
    'extern "C" void std::exit (int) throw (); using std::exit;' \
    'extern "C" void std::exit (int); using std::exit;' \
    'extern "C" void exit (int) throw ();' \
@@ -4509,8 +4509,8 @@ main ()
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext
@@ -19514,7 +19514,7 @@ 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 ();' \