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

etc/systemd/Makefile.am

@@ -1 +0,0 @@
-SUBDIRS = system system-generators

etc/systemd/system-generators/Makefile.am

@@ -1,15 +0,0 @@
-include $(top_srcdir)/config/Rules.am
-
-systemdgenerator_PROGRAMS = \
-	zfs-mount-generator
-
-zfs_mount_generator_SOURCES = \
-	zfs-mount-generator.c
-
-zfs_mount_generator_LDADD = \
-	$(abs_top_builddir)/libzfs.la
-
-zfs_mount_generator_LDFLAGS = -pthread
-
-include $(top_srcdir)/config/CppCheck.am
-CPPCHECKTARGETS += zfs-mount-generator

etc/systemd/system/.gitignore

@@ -1,4 +1,3 @@
 *.service
 *.target
-*.preset
 *.timer

etc/systemd/system/Makefile.am

@@ -1,26 +0,0 @@
-CLEANFILES =
-EXTRA_DIST =
-include $(top_srcdir)/config/Substfiles.am
-
-systemdpreset_DATA = \
-	50-zfs.preset
-
-systemdunit_DATA = \
-	zfs-zed.service \
-	zfs-import-cache.service \
-	zfs-import-scan.service \
-	zfs-mount.service \
-	zfs-share.service \
-	zfs-volume-wait.service \
-	zfs-import.target \
-	zfs-volumes.target \
-	zfs.target \
-	zfs-scrub-monthly@.timer \
-	zfs-scrub-weekly@.timer \
-	zfs-scrub@.service
-
-SUBSTFILES += $(systemdpreset_DATA) $(systemdunit_DATA)
-
-install-data-hook:
-	$(MKDIR_P) "$(DESTDIR)$(systemdunitdir)"
-	ln -sf /dev/null "$(DESTDIR)$(systemdunitdir)/zfs-import.service"