How to modify and build a rosbuild package from source in ros groovy?

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I haven't done this for rosfalcon (and can't find any information about it on the wiki), but this has worked for me:

cd $catkin_ws/src
git clone http://<blah blah blah>
cd $catkin_ws
catkin_make
roscd <package>

Where $catkin_ws is an already existing catkin workspace, whose setup.bash file I've already sourced. I'd guess that you either missed the catkin_make or sourcing the workspace's setup file.

Edit: Thanks for the link! After downloading the code via:

bzr branch http://bazaar.launchpad.net/~cyphylab/cyphy/bonsai

It became clear that the problem is this isn't a catkin-ized package. Check this out for information about using the old-style rosbuild system with catkin. In short, make sure you have the bonsai directory somewhere in your ROS_PACKAGE_PATH, after which you should be able to

rosmake cyphy_rosfalcon

Unfortunately, this gives errors on my system (I'm using OSX), but this should at least solve your build problems.