diff --git a/Library/Homebrew/build.rb b/Library/Homebrew/build.rb
index 271e7f141f..bad624afca 100755
--- a/Library/Homebrew/build.rb
+++ b/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?