Honor LIBUSB_LIBS when set by user

LIBUSB_LIBS was not passed to the LDADD variable for the Hamlib test
utilities.  This fix permits the user invoking 'configure' to specify a
static libusb to include the libusb symbols in libhamlib:

	./configure LIBUSB_LIBS="/usr/lib/i386-linux-gnu/libusb.a"

Note that the actual path will vary from system to system.