Fixed configure.sh to set libusb properly on Darwin
Fixed configure.sh to set libusb properly on Darwin

Previously it only set libusb when uname returned "Linux" for the OS, and that
caused the Makefile to try to build USB support, since it only _fails_ to
build USB support when libusb="none".

Now libusb="none" for Darwin. For now.

nickhen/muforth,nickhen/muforth