Hack so that ENV['CFLAGS'] += "foo" always works

So many formula assume CFLAGS etc. are not nil. One fix would be to set them to "" but this would set them in the environment, and that could have consequences for build-scripts. This hack works but with a (hopefully) small caveat.

Fixes Homebrew/homebrew#14580.

Library/Homebrew/superenv.rb

@@ -208,6 +208,26 @@ class << ENV
     [$1.to_i, 1].max
   end
 
+  # Many formula assume that CFLAGS etc. will not be nil.
+  # This should be a safe hack to prevent that exception cropping up.
+  # Main consqeuence of this is that ENV['CFLAGS'] is never nil even when it
+  # is which can break if checks, but we don't do such a check in our code.
+  def [] key
+    if has_key? key
+      fetch(key)
+    elsif %w{CPPFLAGS CFLAGS LDFLAGS}.include? key
+      class << (a = "")
+        attr :key, true
+        def + value
+          ENV[key] = value
+        end
+        alias_method '<<', '+'
+      end
+      a.key = key
+      a
+    end
+  end
+
 end if superenv?