build: expand requirements recursively when modifying ENV

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

Library/Homebrew/build.rb

@@ -56,7 +56,7 @@ at_exit do
 end
 
 def install f
-  f.requirements.each { |dep| dep.modify_build_environment }
+  f.recursive_requirements.each { |req| req.modify_build_environment }
 
   f.recursive_deps.uniq.each do |dep|
     dep = Formula.factory dep

Library/Homebrew/formula.rb

@@ -440,6 +440,12 @@ class Formula
     end
   end
 
+  def recursive_requirements
+    reqs = recursive_deps.map { |dep| dep.requirements }.to_set
+    reqs << requirements
+    reqs.flatten
+  end
+
 protected
 
   # Pretty titles the command and buffers stdout/stderr