autoconf: use include directives instead of recursing down lib

As a bonus, this also adds zfs-mount-generator (previously undescended
down) and libzstd (not included) to CppCheck

As a bonus bonus, abigail rules work out-of-tree, too

Against current trunk:
  $ diff -U0 ./destdir.listing ~/store/code/zfs/destdir.listing
  -destdir/usr/local/include/libspl/sscanf.h

  $ diff --color -U0 ./zfs-2.1.99.tar.gz.listing ../oot/zfs-2.1.99.tar.gz.listing | grep -v @@ | grep -v /Makefile
  -zfs-2.1.99/config/Abigail.am
  -zfs-2.1.99/lib/libspl/include/util/
  -zfs-2.1.99/lib/libspl/include/util/sscanf.h

  $ diff --color -U0 ./zfs-2.1.99.tar.gz.listing ../oot/zfs-2.1.99.tar.gz.listing | grep -v @@ | grep /Makefile
  -zfs-2.1.99/lib/libavl/Makefile.in
  -zfs-2.1.99/lib/libefi/Makefile.in
  -zfs-2.1.99/lib/libicp/Makefile.in
  -zfs-2.1.99/lib/libnvpair/Makefile.in
  -zfs-2.1.99/lib/libshare/Makefile.in
  -zfs-2.1.99/lib/libspl/include/Makefile.in
  -zfs-2.1.99/lib/libspl/include/os/freebsd/Makefile.am
  -zfs-2.1.99/lib/libspl/include/os/freebsd/Makefile.in
  -zfs-2.1.99/lib/libspl/include/os/freebsd/sys/Makefile.am
  -zfs-2.1.99/lib/libspl/include/os/freebsd/sys/Makefile.in
  -zfs-2.1.99/lib/libspl/include/os/linux/Makefile.am
  -zfs-2.1.99/lib/libspl/include/os/linux/Makefile.in
  -zfs-2.1.99/lib/libspl/include/os/linux/sys/Makefile.am
  -zfs-2.1.99/lib/libspl/include/os/linux/sys/Makefile.in
  -zfs-2.1.99/lib/libspl/include/os/Makefile.am
  -zfs-2.1.99/lib/libspl/include/os/Makefile.in
  -zfs-2.1.99/lib/libspl/include/rpc/Makefile.am
  -zfs-2.1.99/lib/libspl/include/rpc/Makefile.in
  -zfs-2.1.99/lib/libspl/include/sys/dktp/Makefile.am
  -zfs-2.1.99/lib/libspl/include/sys/dktp/Makefile.in
  -zfs-2.1.99/lib/libspl/include/sys/Makefile.am
  -zfs-2.1.99/lib/libspl/include/sys/Makefile.in
  -zfs-2.1.99/lib/libspl/include/util/Makefile.am
  -zfs-2.1.99/lib/libspl/include/util/Makefile.in
  -zfs-2.1.99/lib/libspl/Makefile.in
  -zfs-2.1.99/lib/libtpool/Makefile.in
  -zfs-2.1.99/lib/libunicode/Makefile.in
  -zfs-2.1.99/lib/libuutil/Makefile.in
  -zfs-2.1.99/lib/libzfsbootenv/Makefile.in
  -zfs-2.1.99/lib/libzfs_core/Makefile.in
  -zfs-2.1.99/lib/libzfs/Makefile.in
  -zfs-2.1.99/lib/libzpool/Makefile.in
  -zfs-2.1.99/lib/libzstd/Makefile.in
  -zfs-2.1.99/lib/libzutil/Makefile.in
  -zfs-2.1.99/lib/Makefile.in

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

lib/libspl/include/sys/Makefile.am

@@ -1,47 +0,0 @@
-SUBDIRS = dktp
-
-libspldir = $(includedir)/libspl/sys
-libspl_HEADERS = \
-	acl.h \
-	acl_impl.h \
-	callb.h \
-	cmn_err.h \
-	cred.h \
-	debug.h \
-	dkio.h \
-	dklabel.h \
-	feature_tests.h \
-	int_limits.h \
-	int_types.h \
-	inttypes.h \
-	isa_defs.h \
-	kmem.h \
-	kstat.h \
-	list.h \
-	list_impl.h \
-	mhd.h \
-	mkdev.h \
-	policy.h \
-	poll.h \
-	priv.h \
-	processor.h \
-	sha2.h \
-	simd.h \
-	stack.h \
-	stdtypes.h \
-	string.h \
-	stropts.h \
-	sunddi.h \
-	systeminfo.h \
-	time.h \
-	trace_spl.h \
-	trace_zfs.h \
-	types32.h \
-	types.h \
-	tzfile.h \
-	uio.h \
-	va_list.h \
-	varargs.h \
-	vnode.h \
-	wmsum.h \
-	zone.h

lib/libspl/include/sys/dktp/Makefile.am

@@ -1,4 +0,0 @@
-libspldir = $(includedir)/libspl/sys/dktp
-libspl_HEADERS = \
-	fdisk.h
-