man: Fix checking manpages without a full build

Some of the man pages (ex: `man/man8/zed.8`) are generated from `.in`
files, and `zed.8.in` *was* a dependency of `zed.8`, but `zed.8` was
not a dependency of `mancheck-...zed.8`. This usually worked anyways
because a full build had already been run, now it works regardless.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shelvacu on fw <git@shelvacu.com>
Closes #18346

man/Makefile.am

@@ -131,17 +131,18 @@ SUBSTFILES += $(nodist_man_MANS)
 
 MANFILES = $(dist_noinst_man_MANS) $(dist_man_MANS) $(nodist_man_MANS)
 
-PHONY += mancheck
-
 mancheck_verbose = $(mancheck_verbose_@AM_V@)
 mancheck_verbose_ = $(mancheck_verbose_@AM_DEFAULT_V@)
-mancheck_verbose_0 = @echo MANCHECK $(_MTGT);
+mancheck_verbose_0 = @echo MANCHECK $<;
 
-_MTGT = $(subst ^,/,$(subst mancheck-,,$@))
-mancheck-%:
-	$(mancheck_verbose)scripts/mancheck.sh $(_MTGT)
+MANCHECK_TARGETS = $(foreach manfile, $(MANFILES), $(addprefix mancheck-,$(manfile)))
 
-mancheck: $(foreach manfile, $(MANFILES), $(addprefix mancheck-,$(subst /,^,$(manfile))))
+PHONY += $(MANCHECK_TARGETS) mancheck
+
+$(MANCHECK_TARGETS): mancheck-%: %
+	$(mancheck_verbose)scripts/mancheck.sh $<
+
+mancheck: $(MANCHECK_TARGETS)
 
 CHECKS += mancheck