Add stdlib.h for exit command to fix CC=g++ configuration

Signed-off-by: Michael Sartain <mikesart@gmail.com>
Signed-off-by: Bob Rossi <bob@brasko.net>

config/readline_check_version.m4

@@ -67,6 +67,7 @@ LDFLAGS="$LDFLAGS -L${ac_cv_rl_libdir}"
 AC_CACHE_VAL(ac_cv_rl_version,
 [AC_TRY_RUN([
 #include <stdio.h>
+#include <stdlib.h>
 #include <readline/readline.h>
 
 main()