Determine bitness on osx

This reads the bitness from mono which is in path.

src/CppParser/Bindings/premake4.lua

@@ -29,7 +29,14 @@ project "CppSharp.Parser.CSharp"
   if os.is_windows() then
       files { "CSharp/i686-pc-win32-msvc/**.cs" }
   elseif os.is_osx() then
-      files { "CSharp/i686-apple-darwin12.4.0/**.cs" }
+      local file = io.popen("lipo -info `which mono`")
+      local output = file:read('*all')
+      if string.find(output, "x86_64") then  
+        files { "CSharp/x86_64-apple-darwin12.4.0/**.cs" }
+      else
+        files { "CSharp/i686-apple-darwin12.4.0/**.cs" }
+      end
+
   elseif os.is_linux() then
       files { "CSharp/x86_64-linux-gnu/**.cs" }
   else