Squid: warn if bdb 5.x is installed

Formula/squid.rb

@@ -1,10 +1,27 @@
 require 'formula'
 
+class NoBdb5 < Requirement
+  def message; <<-EOS.undent
+    This software can fail to compile when Berkeley-DB 5.x is installed.
+    You may need to try:
+      brew unlink berkeley-db
+      brew install dsniff
+      brew link berkeley-db
+    EOS
+  end
+  def satisfied?
+    f = Formula.factory("berkeley-db")
+    not f.installed?
+  end
+end
+
 class Squid < Formula
   url 'http://www.squid-cache.org/Versions/v3/3.1/squid-3.1.9.tar.bz2'
   homepage 'http://www.squid-cache.org/'
   md5 '896ace723445ac168986ba8854437ce3'
 
+  depends_on NoBdb5.new
+
   def install
     system "./configure", "--disable-debug", "--disable-dependency-tracking",
                           "--prefix=#{prefix}",