Fix X11 proxy constant lookup under 1.9+

Library/Homebrew/requirements.rb

@@ -109,10 +109,19 @@ class X11Dependency < Requirement
   class Proxy < self
     PACKAGES = [:libpng, :freetype, :fontconfig]
 
-    def self.for(name, *tags)
+    class << self
+      def defines_const?(const)
+        if ::RUBY_VERSION >= "1.9"
+          const_defined?(const, false)
+        else
+          const_defined?(const)
+        end
+      end
+
+      def for(name, *tags)
         constant = name.capitalize
 
-      if const_defined?(constant)
+        if defines_const?(constant)
           klass = const_get(constant)
         else
           klass = Class.new(self) do
@@ -125,6 +134,7 @@ class X11Dependency < Requirement
       end
     end
   end
+end
 
 
 # There are multiple implementations of MPI-2 available.