autoconf: use include directives instead of recursing down etc

dist diff:

-zfs-2.1.99/etc/systemd/system/50-zfs.preset.in
+zfs-2.1.99/etc/systemd/system/50-zfs.preset

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13316