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 build a static /bin/mksh.

Pass DIET=yes to the script to use dietlibc instead of glibc. This
requires the optional dependency dietlibc. DIET=yes builds are always
static; no need for STATIC=yes. Cannot be combined with MUSL=yes.

Pass MUSL=yes to the script to use musl instead of glibc. This
requires the optional dependency musl. Can be combined with
STATIC=yes. Cannot be combined with DIET=yes.