This is mksh, the "MirBSD Korn Shell" (or "MidnightBSD Korn Shell").
It is a successor to the Public Domain Korn Shell (pdksh)

Pass STATIC=yes to the script to create a static build.

Pass DIET=yes to the script in order to use dietlibc (this
requires the optional dependency dietlibc)

Pass MUSL=yes to the script in order to use musl (this
requires the optional dependency musl)