Allow to build from the top-level folder
Allow to build from the top-level folder

This simplifies usage with packaging or build automation systems, in
particular I ran into this with Flatpak.

This isn't particularly elegant, but it does have the advantage that the
current sub-folder builds are entirely unaffected.

nu-book/zxing-cpp,huycn/zxing-cpp,huycn/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,nu-book/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp