Recursively scan for keg-only deps before generating build ENV

Remove cairo dep from gtk formula as a consequence.

Library/Homebrew/build.rb

@@ -40,7 +40,7 @@ at_exit do
 end
 
 def install f
-  f.deps.uniq.each do |dep|
+  f.recursive_deps.uniq.each do |dep|
     dep = Formula.factory dep
     if dep.keg_only?
       ENV.prepend 'LDFLAGS', "-L#{dep.lib}"