Historically this is a portability fix. Since openzfs has its own implementation of function `getexecname()` in `libspl`, it is not needed any more.
Also without this `libspl` will end up incompatible version of `getprogname()` with standard lib c. In particular, if zfs is enabled, programs such as `pgrep` in `/rescue` will be statically linked with the wrong version and will not function properly.
PR: [[https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=269738 |269738]]
Fixes: 9e5787d2284e Merge OpenZFS support in to HEAD