gencpp: Setup python script path during runtime via find_program

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>