Create a `libnormaliz_julia_jll` which contains a compiled version of the code in `deps/src/`

[fingolfin]

... just like there are libsingular_julia_jll and libpolymake_julia_jll, see

• https://github.com/JuliaBinaryWrappers/libpolymake_julia_jll.jl is created from https://github.com/JuliaPackaging/Yggdrasil/tree/master/L/libpolymake_julia
• https://github.com/JuliaBinaryWrappers/libsingular_julia_jll.jl is created from https://github.com/JuliaPackaging/Yggdrasil/tree/master/L/libsingular_julia

Of course this is not strictly necessary, but on the long run it improves the user experience dramatically: users don't need a C++ compiler, there are no ABI problems, etc.

However, I'd do it a bit differently than what we did for the two aforementioned JLLs: they moved the C++ sources into separate repositories. With what I know now, I think this was a mistake. The source should stay in this repository.

Also, there should be a way to easily test updates to the C++ sources w/o having to do complicated things with the JLL. This can be done, and I've done it with GAP.jl. Roughly the idea is to record in libnormaliz_julia_jll a checksum of the source code used to make it; if the source code diverges, then fall back to using the local C++ code (so one has to compile it etc. -- but this is only relevant for developers of this package, who can be expected to have a compiler around etc.). I can provide additional details on how to do this if wanted.