setup_once.h: tighten requirements for stdbool.h header inclusion

Include stdbool.h only when it is available and configure is capable of
detecting a proper 'bool' data type when the header is included.

Compilation fix for old or unpatched versions of XL C compiler.

Report: http://curl.haxx.se/mail/archive-2012-04/0022.html

lib/setup_once.h

@@ -79,7 +79,7 @@
 #include <fcntl.h>
 #endif
 
-#ifdef HAVE_STDBOOL_H
+#if defined(HAVE_STDBOOL_H) && defined(HAVE_BOOL_T)
 #include <stdbool.h>
 #endif