zsh: disable /etc files by default

Closes Homebrew/homebrew#24538.

Formula/zsh.rb

@@ -9,7 +9,7 @@ class Zsh < Formula
   depends_on 'gdbm'
   depends_on 'pcre'
 
-  option 'disable-etcdir', 'Disable the reading of Zsh rc files in /etc'
+  option 'enable-etcdir', 'Enable the reading of Zsh rc files in /etc'
 
   def install
     args = %W[
@@ -26,7 +26,11 @@ class Zsh < Formula
       --with-tcsetpgrp
     ]
 
-    args << '--disable-etcdir' if build.include? 'disable-etcdir'
+    if build.include? 'enable-etcdir'
+      args << '--enable-etcdir'
+    else
+      args << '--disable-etcdir'
+    end
 
     system "./configure", *args
 
@@ -50,14 +54,6 @@ class Zsh < Formula
   def caveats; <<-EOS.undent
     To use this build of Zsh as your login shell, add it to /etc/shells.
 
-    If you have administrator privileges, you must fix an Apple miss
-    configuration in Mac OS X 10.7 Lion by renaming /etc/zshenv to
-    /etc/zprofile, or Zsh will have the wrong PATH when executed
-    non-interactively by scripts.
-
-    Alternatively, install Zsh with /etc disabled:
-      brew install --disable-etcdir zsh
-
     Add the following to your zshrc to access the online help:
       unalias run-help
       autoload run-help