Slight improvements to the build system

[viroulep]

Here are a few fixes to the issues I encountered when building the project:

• nlohmann's json library has a quite huge repository, since the goal is not to make changes to the dependencies it's probably fine to shallow clone them.
• when using gcc/clang with an (old?) install of libstd++ (shipped with gcc 8), adding stdc++fs is necessary for the project to link properly. I'm not sure if it's still needed in more recent versions.
• when compiling projects linking with LLVM, I think -fno-rtti is necessary because LLVM builds without RTTI.

[JPenuchot]

Sorry I missed the notification for this PR. I pushed these modifications to the project myself since it has changed a lot. Thank you!