Support for CMake find_package(…)

[tanius]

So-called "modern" CMake allows to use CMake packages. That enabled one CMake build system (= one call to cmake) to rely on libraries etc. provided and exported by other build systems, located in other repositories on the file system or even installed from operating system packages. QZXingNu already uses that for its dependencies, but does not yet provide a package itself.

It would be great if QZXingNu could be provided as such a CMake package, simplifying the use for those who don't want to include it into their source tree to then use add_subdirectory(…). See: cmake-packages(7) → Creating packages.