cmake: specify explicit c++-17 target_compile_feature for ReaderTest
cmake: specify explicit c++-17 target_compile_feature for ReaderTest

Drop 'global' setting of CMAKE_CXX_STANDARD in subproject(blackbox) as
this approach seems not to work in a VisualStudio build environment.

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