By statically linking `libgmp` on MacOS, we can avoid problems we have with OpenSSL, including dynamic linking (Fixes#110), and certificate issues (Fixes#170)
Unfortunately, it makes fnm fail to build on Windows, which is not that of a big deal. I want to say thanks for @ulrikstrid for all the work he invested into making the build pass on Windows, but I think that implementing fnm on Windows will be better off as a separate program — maybe even in this very repo, while sharing the feature tests with the Unix implementation