NOTFORMERGE: Generate and install a CMake config file

This set of files allows CMake-based projects to detect libusb without
needing to implement special scripts or piggybacking on pkg-config.

Now it is possible to use find_package(libusb) in CMake scripts, which, in
the CMake worls, is the recommended way to find a dependency.

NOTFORMERGE: This is only partially tested.