gem pristine pg error fails

Question

$ gem pristine pg --version 0.14.1 pg --version 0.12.2
Restoring gems to pristine condition...
Building native extensions.  This could take a while...
ERROR:  While executing gem ... (Gem::Installer::ExtensionBuildError)
ERROR: Failed to build gem native extension.

    /Users/macbook/.rvm/rubies/ruby-1.9.3-p374/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
--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/macbook/.rvm/rubies/ruby-1.9.3-p374/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/macbook/.rvm/gems/ruby-1.9.3-p374/gems/pg-0.12.2 for inspection.
Results logged to /Users/macbook/.rvm/gems/ruby-1.9.3-p374/gems/pg-0.12.2/ext/gem_make.out

Answer 1

$ gem pristine pg --version 0.14.1 pg --version 0.12.2 -- --with-pg-config=/Library/PostgreSQL/9.1/bin/pg_config
Restoring gems to pristine condition...
Building native extensions.  This could take a while...
Restored pg-0.12.2
Building native extensions.  This could take a while...
Restored pg-0.12.2