cmake: Fixes and cleanup for libusb detection and version checks

The version checks are now enforced. (Fixes #130)

FindLibUsb now warns if the user is using < libusb-1.0.16, as this or
1.0.17 are *highly* recommended.

The redundant version check in Windows (ver.c) has been removed. Testing
with CMake 2.8.11.2 has shown check_library_exists() to be sufficient.