Requirement: add modify_build_environment method

Rather than doing type introspection in build.rb, just define a method
to perform the necessary environment setup for Requirements.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

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

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