You don't. Watch:

  wget http://www.php.net/get/php-5.3.16.tar.gz/from/us2.php.net/mirror -O php-5.3.16.tar.gz
  tar zxvf php-5.3.16.tar.gz
  cd php-5.3.16/ext/pcntl
  phpize
  ./configure
  make
  sudo make install

At this point you should see a pcntl.so in your extension_dir. Add an extension=pcntl.so to your php.ini file and you are good to go.

Of course with non-bundled extensions from pecl or github you can skip the tarball step.