games/openenroth: unbreak the port's build in pre-C++20 environments

Pull Clang 13 when it's not available in the base (via USES+=llvm:13)
and use Boost's ``unordered_map'' implementation while the system one
is insufficient; avoid ``std::u8string'' for the same reason.

Set LLD_UNSAFE to unbreak linking against OpenAL on i386 (ld: error:
cannot preempt symbol: alGetError).

While here, slightly wordsmith the installation message.

Reported by: pkg-fallout