build: use the "force" mechanism to always run the revision rule

Whenever 'make' is run, it should check whether the revision has
changed, and if so, it should update src/revision.h.

This portable FORCE mechanism should have been the replacement for
the phony target that was removed in a33a4f00, five commits ago.

This fixes https://savannah.gnu.org/bugs/?50956 in a better way.

src/Makefile.am

@@ -12,10 +12,13 @@ endif
 nano.o: revision.h
 winio.o: revision.h
 
-revision.h:
+# Always run the commands in this rule.  But update the target
+# only when the revision actually changed.
+revision.h: FORCE
 	@[ -f $@ ] || touch $@
 	@echo "#define $(SOMETHING)" | cmp -s $@ - || \
 			echo "#define $(SOMETHING)" > $@
+FORCE:
 
 bin_PROGRAMS = 	nano
 nano_SOURCES =	browser.c \