As previously announced, execute order 66: https://lists.freedesktop.org/archives/systemd-devel/2022-September/048352.html The meson options split-usr, rootlibdir and rootprefix become no-ops that print a warning if they are set to anything other than the default values. We can remove them in a future release.
The name "def.h" originates from before the rule of "no needless abbreviations" was established. Let's rename the file to clarify that it contains a collection of various semi-related constants.