graphviz: force build with llvm instead of clang

(Same technique being used for Qt.)

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/graphviz.rb

@@ -34,6 +34,10 @@ class Graphviz < Formula
     args << "--disable-swig" unless build_bindings?
     args << "--without-pangocairo" unless ARGV.include? '--with-pangocairo'
 
+    # Compilation currently fails with the newer versions of clang
+    # shipped with Xcode 4.3+
+    ENV.llvm if MacOS.clang_version.to_f <= 3.1
+
     system "./configure", *args
     system "make install"