slony: specify --with-pgcofigdir

Fixes #https://github.com/mxcl/homebrew/issues/25016

Formula/slony.rb

@@ -8,7 +8,9 @@ class Slony < Formula
   depends_on :postgresql
 
   def install
+    postgres = Formula.factory('postgresql')
     system "./configure", "--disable-debug",
+                          "--with-pgconfigdir=#{postgres.opt_prefix}/bin",
                           "--prefix=#{prefix}"
     system "make install"
   end