diff --git a/Library/Homebrew/build.rb b/Library/Homebrew/build.rb
index 72628ac462..ef74a1942e 100755
--- a/Library/Homebrew/build.rb
+++ b/Library/Homebrew/build.rb
@@ -56,7 +56,7 @@ at_exit do
 end
 
 def install f
-  ENV.x11 if f.external_deps.any? { |dep| dep.is_a? X11Dependency }
+  f.external_deps.each { |dep| dep.modify_build_environment }
 
   f.recursive_deps.uniq.each do |dep|
     dep = Formula.factory dep
diff --git a/Library/Homebrew/dependencies.rb b/Library/Homebrew/dependencies.rb
index 65026dab80..6a4c3e2286 100644
--- a/Library/Homebrew/dependencies.rb
+++ b/Library/Homebrew/dependencies.rb
@@ -118,6 +118,7 @@ class Requirement
   def satisfied?; false; end
   def fatal?; false; end
   def message; ""; end
+  def modify_build_environment; nil end
 end
 
 
@@ -188,4 +189,8 @@ class X11Dependency < Requirement
     EOS
   end
 
+  def modify_build_environment
+    ENV.x11
+  end
+
 end