Makefile: put script in user-writable directory

[ErikBoesen]

Right now make puts the program into /usr/local/bin. This is common practice, but that directory by default requires users to have root/sudo access.

A better option would be to put it in ~/.local/bin, which is writable without root (by the owner of the home directory, of course).

The issue with this is that the $PATH variable doesn't contain that directory by default on most systems. We'd need to check that that directory is in the path and warn the user if necessary that they need to add it.