Avoid hardcoding /usr/local in lmicdusb Makefile

[carlocab]

The lmicdiusb Makefile hardcodes /usr/local for Darwin at

lm4tools/lmicdiusb/Makefile

Lines 40 to 41 in 61a7d17

	LIBUSB_CFLAGS := -I/usr/local/include/libusb-1.0
	LIBUSB_LIBDIR := /usr/local/lib

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