yaml-cpp: support C++11 mode.

* Requires dependencies built with C++11 when requesting C++11 mode.

Closes Homebrew/homebrew#16162.

Formula/yaml-cpp.rb

@@ -5,10 +5,19 @@ class YamlCpp < Formula
   url 'http://yaml-cpp.googlecode.com/files/yaml-cpp-0.5.1.tar.gz'
   sha1 '9c5414b4090491e96d1b808fe8628b31e625fdaa'
 
+  option :cxx11
+
   depends_on 'cmake' => :build
-  depends_on 'boost'
+
+  if build.cxx11?
+    depends_on 'boost' => 'c++11'
+  else
+    depends_on 'boost'
+  end
 
   def install
+    ENV.cxx11 if build.cxx11?
+
     system "cmake", ".", *std_cmake_args
     system "make install"
   end