pgpool-ii: use a Requirement for Postgres

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/pgpool-ii.rb

@@ -1,21 +1,32 @@
 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 PgpoolIi < Formula
-  url 'http://pgfoundry.org/frs/download.php/2841/pgpool-II-3.0.1.tar.gz'
   homepage 'http://pgpool.projects.postgresql.org/'
+  url 'http://pgfoundry.org/frs/download.php/2841/pgpool-II-3.0.1.tar.gz'
   md5 '86d8beff0396d11b6753dd2be31bcad7'
 
-  def install
-     if `/usr/bin/which pg_config`.chomp.empty?
-      opoo "No PostgreSQL was detected."
-      puts <<-EOS.undent
-        This formula uses `pg_config` to detect an installed PostgreSQL instead
-        of "depends_on 'postgresql'" so you can use a non-Homebrew version.
-        You may want to `brew install postgresql` if you don't have another
-        version already installed.
-      EOS
-    end
+  depends_on PostgresqlInstalled.new
 
+  def install
     system "./configure", "--prefix=#{prefix}", "--mandir=#{man}"
     system "make install"
   end