openocd binary for Darwin/OSX is built against Homebrew-installed libusb

[nimblemachines]

The pre-built openocd binary (as of commit 9bab0782) for Darwin fails to work if the user doesn't have libusb installed in /usr/local/opt/.

otool -L openocd shows that it has been linked against libusb binaries in /usr/local/opt/, which suggests that the builder used Homebrew and hadn't "sanitized" the build environment to prevent these local-to-their-setup dependencies from creeping into the built binary.

Because the end user may not be using Homebrew, and may very well not have libusb installed, statically linking libusb (which isn't that big) into the binary seems like a simple solution, and will help to forestall confusion.