attempts to fix vsnprintf() detection under HP-UX

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@18808 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/wxchar.h

@@ -594,6 +594,15 @@ WXDLLEXPORT bool wxOKlibc(); // for internal use
 // printf() family saga
 // ----------------------------------------------------------------------------
 
+/*
+   For some systems vsnprintf() exists in the system libraries but not in the
+   headers, so we need to declare it ourselves to be able to use it.
+ */
+#ifndef HAVE_VSNPRINTF_DECL
+    extern "C"
+    int vsnprintf(char *str, size_t size, const char *format, va_list ap);
+#endif // !HAVE_VSNPRINTF_DECL
+
 /*
    First of all, we always want to define safe snprintf() function to be used
    instead of sprintf(). Some compilers already have it (or rather vsnprintf()