Avoid hardcoding `/usr/local` in lmicdusb Makefile

[carlocab]

The lmicdiusb Makefile hardcodes /usr/local for Darwin at

https://github.com/utzig/lm4tools/blob/61a7d17b85e9b4b040fdaf84e02599d186f8b585/lmicdiusb/Makefile#L40-L41

This isn't always where libusb can be found (e.g. if libusb were installed via MacPorts, or on an M1 Mac via Homebrew), so it would be nice if this could be made more flexible.

I can try to find the time to send a patch when I'm less busy. (A new macOS version was recently released, so it's a busy time for Homebrew at the moment.)

Related: Homebrew/homebrew-core#88207