Ensure static libraries have correct mod time even if ranlib messes it up.

In at least Apple's version of ranlib, the output file is updated to have
a mod time equal to the max of the timestamps of its components, and that
data only has seconds precision.  On a filesystem with sub-second file
timestamp precision --- say, APFS --- this can result in the finished
static library appearing older than its input files, which causes useless
rebuilds and possible outright failures in parallel makes.

We've only seen this reported in the field from people using Apple's
ranlib with a non-Apple make, because Apple's make doesn't know about
sub-second timestamps either so it doesn't decide rebuilds are needed.
But Apple's ranlib presumably shares code with at least some BSDen,
so it's not that unlikely that the same problem could arise elsewhere.

To fix, just "touch" the output file after ranlib finishes.

We seem to need this in only one place.  There are other calls of
ranlib in our makefiles, but they are working on intermediate files
whose timestamps are not actually important, or else on an installed
static library for which sub-second timestamp precision is unlikely
to matter either.  (Also, so far as I can tell, Apple's ranlib doesn't
mess up the file timestamp in the latter usage anyhow.)

In passing, change "ranlib" to "$(RANLIB)" in one place that was
bypassing the make macro for no good reason.

Per bug #15525 from Jack Kelly (via Alyssa Ross).
Back-patch to all supported branches.

Discussion: https://postgr.es/m/15525-a30da084f17a1faa@postgresql.org

src/Makefile.shlib

@@ -287,11 +287,17 @@ all-static-lib: $(stlib)
 
 all-shared-lib: $(shlib)
 
+# In this rule, "touch $@" works around a problem on some platforms wherein
+# ranlib updates the library file's mod time with a value calculated to
+# seconds precision.  If the filesystem has sub-second timestamps, this can
+# cause the library file to appear older than its input files, triggering
+# parallel-make problems.
 ifndef haslibarule
 $(stlib): $(OBJS) | $(SHLIB_PREREQS)
 	rm -f $@
 	$(LINK.static) $@ $^
 	$(RANLIB) $@
+	touch $@
 endif #haslibarule
 
 
@@ -450,7 +456,7 @@ install-lib-static: $(stlib) installdirs-lib
 	$(INSTALL_STLIB) $< '$(DESTDIR)$(libdir)/$(stlib)'
 ifeq ($(PORTNAME), darwin)
 	cd '$(DESTDIR)$(libdir)' && \
-	ranlib $(stlib)
+	$(RANLIB) $(stlib)
 endif
 
 install-lib-shared: $(shlib) installdirs-lib