Patches to enable building libirecovery without libusb on OS X.

I don't have much experience with autoconf, so please review my changes to configure.ac. And, I do not have an installation of Linux, so please confirm that it builds.

On Nov 14th, I sent an email to the list with the subject "libirecovery without libusb on OS X" suggesting a cleaner way to write the conditional directives, which may be worth considering.