Hide the cxxstdlib data structure better

Library/Homebrew/cxxstdlib.rb

@@ -35,7 +35,7 @@ class CxxStdlib
   end
 
   def check_dependencies(formula, deps)
-    unless formula.class.cxxstdlib.include? :skip
+    unless formula.skip_cxxstdlib_check?
       deps.each do |dep|
         # Software is unlikely to link against anything from its
         # buildtime deps, so it doesn't matter at all if they link

Library/Homebrew/formula.rb

@@ -247,6 +247,10 @@ class Formula
     self.class.skip_clean_paths.include? to_check
   end
 
+  def skip_cxxstdlib_check?
+    self.class.cxxstdlib.include?(:skip)
+  end
+
   # yields self with current working directory set to the uncompressed tarball
   def brew
     validate_attributes :name, :version