Add zfs_gitrev.h to the distributed sources

Commit 109d2c9310 ("Move zfs_gitrev.h to build directory") stopped
distributing zfs_gitrev.h, as it is a generated file. Add it back, with
some changes in behavior.

Change the logic for gitrev as follows
- if the source tree is a git repository, the behavior for build is
  unchanged. For make dist, append -dist to the git tag in the
  distributed version of zfs_gitrev.h.
- otherwise, check if the source tree contains zfs_gitrev.h, and use it
  if so, falling back to "unknown" if it doesn't exist.
- clean it only in make maintainer-clean, so we don't remove it from the
  source tree on make clean or make distclean.

This allows disted sources to track what git tag they originally came
from, with the -dist suffix indicating that the code wasn't built
directly from git and so might contain additional changes beyond the git
tag.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10595

Makefile.am

@@ -50,7 +50,6 @@ EXTRA_DIST += module/zfs/THIRDPARTYLICENSE.cityhash.descrip
 @CODE_COVERAGE_RULES@
 
 GITREV = include/zfs_gitrev.h
-CLEANFILES = $(GITREV)
 
 PHONY = gitrev
 gitrev:
@@ -59,6 +58,9 @@ gitrev:
 all: gitrev
 
 # Double-colon rules are allowed; there are multiple independent definitions.
+maintainer-clean-local::
+	-$(RM) $(GITREV)
+
 distclean-local::
 	-$(RM) -R autom4te*.cache build
 	-find . \( -name SCCS -o -name BitKeeper -o -name .svn -o -name CVS \
@@ -76,6 +78,7 @@ all-local:
 	    ${top_builddir}/scripts/zfs-tests.sh -c
 
 dist-hook:
+	$(AM_V_GEN)$(top_srcdir)/scripts/make_gitrev.sh -D $(distdir) $(GITREV)
 	$(SED) ${ac_inplace} -e 's/Release:[[:print:]]*/Release:      $(RELEASE)/' \
 		$(distdir)/META