Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    /Users/kojimadaiki/.rvm/rubies/ruby-2.0.0-p645/bin/ruby -r ./siteconf20160104-7569-2gpugj.rb 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=/Users/kojimadaiki/.rvm/rubies/ruby-2.0.0-p645/bin/ruby
    --with-pg
    --without-pg
    --enable-windows-cross
    --disable-windows-cross
    --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}/

extconf failed, exit code 1

Gem files will remain installed in /Users/kojimadaiki/.rvm/gems/ruby-2.0.0-p645/gems/pg-0.18.4 for inspection.
Results logged to /Users/kojimadaiki/.rvm/gems/ruby-2.0.0-p645/extensions/x86_64-darwin-14/2.0.0/pg-0.18.4/gem_make.out
An error occurred while installing pg (0.18.4), and Bundler cannot continue.
Make sure that `gem install pg -v '0.18.4'` succeeds before bundling.

githubからフォークしたファイルをbundle installするとたまに上記のようなエラーが表示されます。

gem install pg -v '0.18.4'を打ってみてもエラーが続くのですが、どうしたらよいのでしょうか？