Check whether we have -lusb-1.0 and it has libusb_init().

Don't just check for the libusb header file - apparently at least some
Linux distributions can't install both the 32-bit and 64-bit developer
versions of the libusb library, so you might have the libusb header, and
the library for one bit-width, but not the library for the other
bit-width, so the configure would succeed for the other bit-width but
the build would fail.
Check for the libusb header *and* library.

Belt and suspenders, just in case some Linux distribution rudely
surprises us.