libpq-fe.hが見つからない

Mac上のRailsでPostgresQLを使おうと思って、以下のエラー。

$ sudo gem install pg
Building native extensions.  This could take a while...
ERROR:  Error installing pg:
	ERROR: Failed to build gem native extension.

/Users/rish/.rvm/rubies/ruby-1.9.2-p180/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/rish/.rvm/rubies/ruby-1.9.2-p180/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/rish/.rvm/gems/ruby-1.9.2-p180/gems/pg-0.11.0 for inspection.
Results logged to /Users/rish/.rvm/gems/ruby-1.9.2-p180/gems/pg-0.11.0/ext/gem_make.out

$ env ARCHFLAGS="-arch x86_64" gem install pg -- --with-pg-dir=/Library/PostgreSQL/9.0/

--with-pg-dirでPostgresのインストールしてあるディレクトリを指定してやればよい。