title: gem install pg --with-pg-config works, bundle fails
body: <p>WhenI run (as root) </p>
  
  <pre><code>gem install pg -v '0.12.0' -- --with-pg-config=/usr/pgsql-9.1/bin/pg_config
  </code></pre>
  
  <p>I get the following output: </p>
  
  <pre><code>#-&gt; gem instal pg -v '0.12.0' -- --with-pg-config=/usr/pgsql-9.1/bin/pg_config
  Building native extensions.  This could take a while...
  Successfully installed pg-0.12.0
  1 gem installed
  Installing ri documentation for pg-0.12.0...
  Installing RDoc documentation for pg-0.12.0...
  #-&gt; 
  </code></pre>
  
  <p>When I run bundle install: </p>
  
  <pre><code>Installing pg (0.12.0) with native extensions 
  Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
  
      /usr/local/rvm/rubies/ruby-1.9.2-p290/bin/ruby extconf.rb 
  checking for pg_config... no
  No pg_config... trying anyway. If building fails, please try again with
  --with-pg-config=/path/to/pg_config
  checking for libpq-fe.h... no
  Can't find the 'libpq-fe.h header
  *** extconf.rb failed ***
  Could not create Makefile due to some reason, probably lack of
  necessary libraries and/or headers.  Check the mkmf.log file for more
  details.  You may need configuration options.
  
  Provided configuration options:
  --with-opt-dir
  --with-opt-include
  --without-opt-include=${opt-dir}/include
  --with-opt-lib
  --without-opt-lib=${opt-dir}/lib
  --with-make-prog
  --without-make-prog
  --srcdir=.
  --curdir
  --ruby=/usr/local/rvm/rubies/ruby-1.9.2-p290/bin/ruby
  --with-pg
  --without-pg
  --with-pg-dir
  --without-pg-dir
  --with-pg-include
  --without-pg-include=${pg-dir}/include
  --with-pg-lib
  --without-pg-lib=${pg-dir}/lib
  --with-pg-config
  --without-pg-config
  --with-pg_config
  --without-pg_config
  
  
  Gem files will remain installed in /var/www/simpletrac/vendor/cache/ruby/1.9.1/gems/pg-    0.12.0 for inspection.
  Results logged to /var/www/simpletrac/vendor/cache/ruby/1.9.1/gems/pg-0.12.0/ext/gem_make.out
  An error occured while installing pg (0.12.0), and Bundler cannot continue.
  Make sure that `gem install pg -v '0.12.0'` succeeds before bundling.
  </code></pre>
  
  <p>I have libpq-fe.h installed at /usr/pgsql-9.1/include/libpq-fe.h. So, I tried </p>
  
  <pre><code>gem install pg -v '0.12.0' -- --with-pg-config=/usr/pgsql-9.1/bin/pg_config --with-pg-lib=/usr/pgsql-9.1/include/libpq-fe.h but still no go. 
  </code></pre>
  
  <p>Any help would be greatly appreciated.</p>
  
  <p>Also, I have installed postgresql91-devel and ruby-devel.  Running CentOS 6.</p>