Make sure to pass all variables defining the proper python version as found by
robotpkg, otherwise ros CMakeFiles may happily find python-3.2 and use it
instead of python<3, although this does not work in the end (at least ros-rx
does this and breaks on F17).

This is done in the common ros Makefile as this affects all packages.

Factor out the following:
 - PKGNAME with ros- prefix. This is overrided for a few base pkgs only
 - MAINTAINER
 - LICENSE

These can still be overriden on a per pkg basis.

Project genmsg exists in order to decouple code generation from ROS .msg & .srv
format files from the parsing of these files and from impementation details of
the build system (project directory layout, existence or nonexistence of
utilities like rospack, values of environment variables such as
ROS_PACKAGE_PATH): i.e. none of these are required to be set in any particular
way.