Makefile: get binary paths from user's $PATH

After weighing the pros and cons of the two, it seems that it would be
somewhat better to have these be found via the $PATH variable, rather
than full paths being specified. I don't know if people will like this,
or if it's even a good idea (some may still have to specify the
variables anyway), but It's going into a spearate commit.
Change the MANAGED dir definition

If someone has the `Managed` directory outside of their KSP installation
for some weird reason, or has a different KSP install...I don't actually
know. Just changing the way ${MANAGED} is set, really.