Fix the cross-build after recent commits

• Provide a sys/md4.h in the cross-build environment to fix bootstrap of libmd.
• flua now exposes WTRAPPED which isn't incredibly common- make it conditional, we probably won't be using it in any bootstrap context any time soon.

Fixes: 442e0975ee4b3 ("Consolidate md4 implementations written in C")
Fixes: c2caf3b3313fe ("flua: posix: add more useful functions [...]")