Mac OSX Mountain Lion 上安裝 postgres gems 問題

替 mac 上了 Mountain Lion，然後想在上面繼續 ruby on rails development，可是在行 bundle install 時出現安裝不了 pg 的訊息：

Installing pg (0.13.2) with native extensions 
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

        /Users/ryan/.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
    --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=/Users/ryan/.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 /Users/ryan/.rvm/gems/ruby-1.9.2-p290/gems/pg-0.13.2 for inspection.
Results logged to /Users/ryan/.rvm/gems/ruby-1.9.2-p290/gems/pg-0.13.2/ext/gem_make.out
An error occured while installing pg (0.13.2), and Bundler cannot continue.
Make sure that `gem install pg -v '0.13.2'` succeeds before bundling.

以為因為沒有裝 postgresql，誰知用 homebrew 安裝 postgresql 後情況沒有任何改善。在網上找了很久，就算裝了 xcode 和 command line for xcode也不行。費了一天後，終於找到解決方法。

原來要裝這個 gcc installer，裝了後便可以用

gem install pg

裝 posgresql gems ！