Add options/install_flags for boost/pcl.

 - as long as https://github.com/ros-infrastructure/rosdep/pull/304
   is not merged these will be ignored. If we merge this before rosdep
   is updated it will make testing that rosdep change slightly more easy.
 - install_flags will be used for `brew install`, but unlike options
   they are not checked for already installed packages
 - for boost --with-python see https://github.com/ros-infrastructure/rosdep/issues/330
 - for pcl --HEAD on Mavericks see
   - http://answers.ros.org/question/173654/cant-install-pcl-with-homebrew-on-osx-109-during-ros-hydro-installation/
   - https://github.com/ros/homebrew-hydro/issues/3
What does this change do? It seems to me the other changes cover the problem?