Add install/uninstall/reinstall rules, and make work on Windows
To make it work on Windows, we list the files with ls, sed out
only files ending with .default, and grabbing the first one.
We add the reinstall target to make testing easier on Windows,
since ln -s on Windows simply copies the files to the new
location. reinstall therefore removes it and copies a fresh
version.
Also switch to simply $(HOME)/.thunderbird/*.default on Unix,
as that's really the standard path, while .mozilla-thunderbird
is an Debian/Ubuntu invention.