doctor: require CLT on 10.9.

Xcode-only setup is badly broken and will remain that way until fixed by
Apple. For many 10.9 issues this fixes things so let's tell the users.

Library/Homebrew/cmd/doctor.rb

@@ -191,7 +191,9 @@ def check_for_broken_symlinks
 end
 
 def check_xcode_clt
-  if MacOS::Xcode.installed?
+  if MacOS.version >= :mavericks
+    __check_clt_up_to_date
+  elsif MacOS::Xcode.installed?
     __check_xcode_up_to_date
   elsif MacOS.version >= :lion
     __check_clt_up_to_date