Add deprecation warning to non-DSL fails_with_llvm

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

Library/Homebrew/compat/compatibility.rb

@@ -75,6 +75,8 @@ class Formula
   # This used to be called in "def install", but should now be used
   # up in the DSL section.
   def fails_with_llvm msg=nil, data=nil
+    opoo "Calling fails_with_llvm in the install method is deprecated"
+    puts "Use the fails_with DSL instead."
     FailsWithLLVM.new(msg, data).handle_failure
   end