HAVE_STDARG_H isn't defined for readline consumers externally so we always
end up using the ancient non-prototypes. Fix that as all compilers we care
about since forever in Gentoo will support this path.

This in particular shows up with rl_message in consumers:
https://bugs.gentoo.org/943820
https://bugs.gentoo.org/944059
https://bugs.gentoo.org/944300
https://bugs.gentoo.org/944344
--- a/rlstdc.h
+++ b/rlstdc.h
@@ -44,7 +44,7 @@
 
 /* Moved from config.h.in because readline.h:rl_message depends on these
    defines. */
-#if defined (__STDC__) && defined (HAVE_STDARG_H)
+#if defined (__STDC__)
 #  define PREFER_STDARG
 #  define USE_VARARGS
 #else