hiredis: fails with clang

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/hiredis.rb

@@ -6,6 +6,14 @@ class Hiredis < Formula
   sha1 'a54dd2b31cb39bc05bf88538c688bf50f8c6c9c7'
   head 'https://github.com/antirez/hiredis.git'
 
+  fails_with :clang do
+    build 318
+    cause <<-EOS.undent
+      argument to 'va_arg' is of incomplete type 'void'
+      This is fixed in HEAD, and can be removed for the next release.
+      EOS
+  end unless ARGV.build_head?
+
   def install
     # Architecture isn't detected correctly on 32bit Snow Leopard without help
     ENV["OBJARCH"] = MacOS.prefer_64_bit? ? "-arch x86_64" : "-arch i386"