Fix to declare vsnprintf prototype with detected condition

It was introduced in HP-UX 10.20 (undocumented) and suppoted in 10.30 and later,
so no need to declare it on HP-UX unconditionally.

Signed-off-by: OBATA Akio <obache@wizdas.com>
(cherry picked from commit 76a04f5fce)

config.h.cmake

@@ -1146,11 +1146,7 @@ __END_DECLS
 
 
 
-/*
- * On HP-UX, the declaration of vsnprintf() is needed every time !
- */
-
-#if !defined(HAVE_VSNPRINTF) || defined(hpux)
+#if !defined(HAVE_VSNPRINTF_PROTO)
 #if __STDC__
 #include <stdarg.h>
 #include <stdlib.h>