Readme instructions for installing `bigint`

[gwerbin]

This is more of a note than an "issue" as such.

I needed to specify CFLAGS='--stdlib=libc++' when installing bigint to silence a warning:

clang: warning: include path for libstdc++ headers not found; pass '-stdlib=libc++' on the command line to use the libc++ standard library instead [-Wstdlibcxx-not-found]

I'm not sure if this is due to some quirk of my own Mac Brew setup, or if it's common to some or all other setups.

[Russoul]

The fact that the bigint library is based on c++ has been a nuisance since the very beginning 😑

So it looks like linking against libc++ is the optimal solution for mac, where
libstdc++ is the linux counterpart.

libstdc++ seems to work for me on mac though, I use nix.

Maybe one just needs to fetch the package that contains libstdc++ via a package-manager of their choice and it will work 🤷

Let me keep this issue open, for future reference.