genlisp: Setup python script path during runtime via find_program

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

This closes #87 with the following error:
*** No rule to make target `/usr/lib/genlisp/gen_lisp.py', needed by
    `devel/share/common-lisp/ros/std_msgs/msg/Bool.lisp'.  Stop

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