title: Bundler tries and fails to install the pg gem, which is already successfully installed
body: <p>I've got the pg gem installed:</p>
  
  <pre><code>$ gem install pg
  Building native extensions.  This could take a while...
  Successfully installed pg-0.15.1
  1 gem installed
  
  $ gem list | grep pg
  pg (0.15.1, 0.14.1)
  </code></pre>
  
  <p>However whenever i try to run 'bundle install' on my rails app, it tries to install pg again, and fails:</p>
  
  <pre><code>Installing pg (0.15.1) with native extensions
  Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension
  checking for pg_config... yes
  Using config values from /usr/bin/pg_config
  checking for libpq-fe.h... yes
  checking for libpq/libpq-fs.h... yes
  checking for pg_config_manual.h... yes
  checking for PQconnectdb() in -lpq... no
  checking for PQconnectdb() in -llibpq... no
  checking for PQconnectdb() in -lms/libpq... no
  Can't find the PostgreSQL client library (libpq)
  ...
  </code></pre>
  
  <p>How can i get bundler to simply use my already-installed version of pg? My Gemfile.lock has:</p>
  
  <pre><code>pg (0.15.1)
  </code></pre>
  
  <p>Which matches the version i can see with gem list. I'm not sure what's wrong here.</p>