To update to the hydro release version, the gencpp patch required
some minor adjustments.

The python script path for gen_cpp.py is hard coded in the CMake
file and doesn't work during cross compile. Setup it via find_program
during runtime.

This closes #91 with the following error:
*** No rule to make target `/usr/lib/gencpp/gen_cpp.py', needed by
`devel/include/roscpp/Logger.h'.  Stop

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>

All ROS packages with install tools in the bin directory or install
python code with is used by other packages should be used as native
version. Adapte the DEPENDS accordingly.

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>