Commit
e3cc3458d40a3129e9d249f4c1370de5f45ccb9e
by falconconfigure.ac: lower libusb-1.0 requirement to 1.0.20
Current configure.ac requires libusb version to be >= 1.0.21.
Slackware 14.2 has libusb version 1.0.20. Lowering the
requirement in configure.ac produces a successful build,
i.e., the actual code does not use any libusb APIs that are
new with version 1.0.21.
Change-Id: I15586757e2985184359a4488c79f2c587affeb56