build-aux/Makefile.am (.version.xml): Only change timestamp on hash change

The timestamp of the file will now only be updated if the hash (version)
_actually_ changes.  This allows this to be used as a target dependency
without forcing a rebuild each and every time.

RELEASES.md

@@ -13,6 +13,18 @@ TAME developers: Add new changes under a "NEXT" heading as part of the
 commits that introduce the changes.  To make a new release, run
 `tools/mkrelease`, which will handle updating the heading for you.
 
+NEXT
+====
+This release contains changes to the build system to accommodate
+liza-proguic's introduction of step-based packages (in place of a monolithic
+`package-dfns.xml`), as well as miscellaneous improvements.
+
+Build System
+------------
+- Only modify `.version.xml` timestamp when hash changes.  This allows
+  its use as a dependency without forcefully rebuilding each and every time.
+
+
 v17.7.0 (2020-12-09)
 ====================
 This release provides tail-call optimizations aimed at the query system in

build-aux/Makefile.am

@@ -190,9 +190,14 @@ c1map: $(dest_c1map)
 %.xml: %.csvo rater/core/vector/table.xmlo
 	rater/tools/csv2xml $< > $@
 
+# This target is always run, but only update the file (and thus its
+# timestamp) if the hash actually changes, so that we do not rebuild any
+# dependencies unnecessarily.
 version: .version.xml
 .version.xml: FORCE
-	git log HEAD^.. -1 --pretty=format:'<version>%h</version>' > .version.xml
+	git log HEAD^.. -1 --pretty=format:'<version>%h</version>' > $@.new
+	cmp $@ $@.new || mv $@.new $@
+	$(RM) $@.new
 
 ui/program.expanded.xml: ui/program.xml $(program_fragments)
 	$(TAME_TS)