compile on older MacOS

[xiaoyue10131748]

Current clang version for MacOS 10.13.6 is 4.2.1 doesn't support either or <experimental/filesystem>. In order to make filesystem API compatible, I add ghc::filesystem by myself as an additional option by referring to (https://stackoverflow.com/questions/49577343/filesystem-with-c17-doesnt-work-on-my-mac-os-x-high-sierra). Here is the source code if ghc:(https://github.com/gulrak/filesystem).

Here are the changes I made:

1. change #include <filesystem> to #include <ghc/filesystem.hpp>; namespace fs =std::filesystem -> namespace fs = ghc::filesystem in ScannerWorkDirManager.cpp and PluginRegistry.cpp
2. add the following things to the CmakeLists.txt

   add_subdirectory(<Your ghc source code path> [Your target binary path])
   target_link_libraries(iblessing ghc_filesystem)