ANSI C89 is an old standard, but it is a capable standard. Subsequent versions, e.g. C99 or C11 did not improve it with features that miniboxtk would really need. Thus the source code of miniboxtk is implemented in C89.
This makes miniboxtk a bit more portable, especially to old UNIX systems or to exotic systems.
Besides C89, miniboxtk also depends on make (no requirement on GNU make), sh (POSIX or classic) and a few common shell utilities. The project uses scconfig (also written in C89) for detecting backends and libc/system features and for generating Makefiles/configs.
Some external libraries used for backends (e.g. SDL2) may depend on C99 or newer. In such case the glue code in the specific miniboxtk backend may also depend on a higher version of the C standard, but this dependency should be minimized.