Look it up if you really care but basically z as a format specifier
often doesn't work in mingw because reasons. Our CI was working by
accident because of meson previously ignoring the CFLAGS variable. Use
our wrapper instead for these places.
The purpose of osdep/io is to provide a wrapper for POSIX I/O functions
on Windows. The last 2 functions are utility functions which don't serve
this purpose.
Move them to a separate file.