title: Cant install the postgresql gem
body: <blockquote>
    <p><code>$ sudo gem install pg Building native extensions.  This could take a
    while... ERROR:  Error installing pg:     ERROR: Failed to build gem
    native extension.</code></p>
  
  <pre><code>/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby
  </code></pre>
    
    <p>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
    <strong>* extconf.rb failed *</strong> 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.</p>
    
    <p>Provided configuration options:</p>
  
  <pre><code>    --with-opt-dir
      --without-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=/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby
    --with-pg
    --without-pg
    --with-pg-config
    --without-pg-config
    --with-pg_config
    --without-pg_config
    --with-pg-dir
    --without-pg-dir
    --with-pg-include
    --without-pg-include=${pg-dir}/include
    --with-pg-lib
    --without-pg-lib=${pg-dir}
  </code></pre>
    
    <p>/ Gem files will remain installed in
    /Library/Ruby/Gems/2.0.0/gems/pg-0.17.1 for inspection. Results logged
    to /Library/Ruby/Gems/2.0.0/gems/pg-0.17.1/ext/gem_make.out</p>
  </blockquote>