OpenLDAP - add installation workaround

OpenLDAP requires BDB 4.x. Added instructions for installing
to the caveats.

Formula/openldap.rb

@@ -11,4 +11,12 @@ class Openldap <Formula
     system "./configure", "--disable-debug", "--disable-dependency-tracking", "--prefix=#{prefix}"
     system "make install"
   end
+
+  def caveats; <<-EOS.undent
+    OpenLDAP depends on berkeley-db 4.x, but Homebrew provides version 5.x,
+    which doesn't work. To work around this, do:
+      $ brew install http://github.com/adamv/homebrew/raw/versions/Library/Formula/berkeley-db4.rb --without-java
+      $ brew install --ignore-dependencies openldap
+    EOS
+  end
 end