saga-core: fix audit warning

Formula/saga-core.rb

@@ -1,14 +1,33 @@
 require 'formula'
 
+class PostgresqlInstalled < Requirement
+  def message; <<-EOS.undent
+    PostgresQL is required to install.
+
+    You can install this with:
+      brew install postgresql
+
+    Or you can use an official installer from:
+      http://www.postgresql.org/
+    EOS
+  end
+  def satisfied?
+    which 'pg_config'
+  end
+  def fatal?
+    true
+  end
+end
+
 class SagaCore < Formula
   homepage 'http://saga-project.org'
   url 'http://download.saga.cct.lsu.edu/saga-core/saga-core-1.6.tgz'
-  md5 'a5cda84bdae1f96646f39fda0aa7db73'
+  sha1 '89cc089b4feb5aae01ae5d371477d2d9a5204f1d'
 
   head 'https://svn.cct.lsu.edu/repos/saga/core/trunk/'
 
   depends_on 'boost'
-  depends_on 'postgresql'
+  depends_on PostgresqlInstalled.new
 
   def install
     system "./configure", "--prefix=#{prefix}",