generate headers in devel space instead of build space

With the upcoming catkin version passing absolute INCLUDE_DIRS to catkin_package() will be allowed (ros/catkin#600). But since your absolute *build* space path is temporary downstream packages will fail to find that folder. (Before any absolute path was ignored but there where use cases where it should be possible.)

Instead this pull request generates the header in the *devel* space which catkin is aware of and assumes that these generated files will be installed by the user (similar as generated message headers).

Please review and test - I haven't even tried to compile the package after the change :wink: This fix should be released in *Groovy*, *Hydro* and *Indigo*.