sqlite3 をインストールしようとすると、下記のエラー。

Installing sqlite3 1.7.3 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
...(snip)...
An error occurred while installing sqlite3 (1.7.3), and Bundler cannot continue.

In Gemfile:
  sqlite3

Could not configure the build properly (pkg_config). Please install either the `pkg-config` utility or the `pkg-config` rubygem.

*** 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.

pkg config をインストールして解決した。

$ brew install pkgconf

参考