genmsg: Update to handle a list of msg include dirs relative

The last patched version assumes that the variable msg include dirs
is only one directory and not a list of drectories. This leads to
erros if generate_message(DIRECTORY ...) occurs twice in the
CMakeLists files.

Make Error at build/devel/share/cmake/...-msg-paths.cmake:2 (if):
| if given arguments:
|
| "NOT" "IS_ABSOLUTE" "" ""

Fixed issue #72.

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