Fix superenv ENV[] hack

We override ENV[] to always return strings under superenv, because
legacy formulae assume that CFLAGS, etc. are non-nil.

However, the current implementation has a bug. If I simply concatenate
ENV['CFLAGS'] with another string, it mutates ENV['CFLAGS']:

  irb> ENV['CFLAGS']
  => ""
  irb> ENV['CFLAGS'] + 'a'
  => "a"
  irb> ENV['CFLAGS']
  => "a"

Instead, let's simply return an empty string if the key doesn't exist.
This is sufficient because the following are equivalent:

  1. ENV['CFLAGS'] += "string"
  2. ENV['CFLAGS'] = ENV['CFLAGS'] + "string"

Library/Homebrew/superenv.rb

@@ -29,15 +29,7 @@ module Superenv
         if has_key? key
           fetch(key)
         elsif %w{CPPFLAGS CFLAGS LDFLAGS}.include? key
-          class << (a = "")
+          self[key] = ""
-            attr_accessor :key
-            def + value
-              ENV[key] = value
-            end
-            alias_method '<<', '+'
-          end
-          a.key = key
-          a
         end
       end
     end