Revert "zsh: disable /etc files by default"

This reverts commit 52fe004dd257ccf105dc6a5fe4b9cd570285e6b7.

Conflicts:
	Library/Formula/zsh.rb

References Homebrew/homebrew#24538.
References Homebrew/homebrew#25719.

Formula/zsh.rb

@@ -9,7 +9,7 @@ class Zsh < Formula
   depends_on 'gdbm'
   depends_on 'pcre'
 
-  option 'enable-etcdir', 'Enable the reading of Zsh rc files in /etc'
+  option 'disable-etcdir', 'Disable the reading of Zsh rc files in /etc'
 
   def install
     args = %W[
@@ -26,10 +26,10 @@ class Zsh < Formula
       --with-tcsetpgrp
     ]
 
-    if build.include? 'enable-etcdir'
-      args << '--enable-etcdir=/etc'
-    else
+    if build.include? 'disable-etcdir'
       args << '--disable-etcdir'
+    else
+      args << '--enable-etcdir=/etc'
     end
 
     system "./configure", *args
@@ -48,6 +48,14 @@ 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