Use clang's libc++ to be consistent with zcashd

[teor2345]

Motivation

I notice you're linking with -lstdc++, i.e. libstdc++, whereas zcashd uses libc++. I'm not sure that matters if you're recompiling the script code, just thought I'd point it out.

Originally posted by @daira in #84 (comment)

Currently zcash_script links with the default C++ library for the current platform, using the link-cplusplus crate.

Specifications

Since Zcash script behaviour is unspecified, we should try to match zcashd's behaviour as much as possible. But the C++ library is unlikely to change consensus-critical behaviour.

Background

Zebra already links with libstdc++ (gcc) via RocksDB on Linux. librocksdb-sys has its own custom logic for choosing the default C++ library for each platform.

Zebra also uses libclang as a build dependency, which depends on libc++ (clang).

We might get better optimisations and smaller binaries by preferring the clang compiler and using its libc++ library. However, this could also make Zebra builds harder to configure on some platforms.