ENV.x11 automatically if depends_on :x11

Library/Homebrew/build.rb

@@ -54,6 +54,8 @@ at_exit do
 end
 
 def install f
+  ENV.x11 if f.external_deps.any? { |dep| dep.is_a? X11Dependency }
+
   f.recursive_deps.uniq.each do |dep|
     dep = Formula.factory dep
     if dep.keg_only?