Fix make rules that generate multiple output files.

 Fix make rules that generate multiple output files.
 
 For years, our makefiles have correctly observed that "there is no correct
 way to write a rule that generates two files".  However, what we did is to
 provide empty rules that "generate" the secondary output files from the
 primary one, and that's not right either.  Depending on the details of
 the creating process, the primary file might end up timestamped later than
 one or more secondary files, causing subsequent make runs to consider the
 secondary file(s) out of date.  That's harmless in a plain build, since
 make will just re-execute the empty rule and nothing happens.  But it's
 fatal in a VPATH build, since make will expect the secondary file to be
 rebuilt in the build directory.  This would manifest as "file not found"
 failures during VPATH builds from tarballs, if we were ever unlucky enough
 to ship a tarball with apparently out-of-date secondary files.  (It's not
 clear whether that has ever actually happened, but it definitely could.)
 
 To ensure that secondary output files have timestamps >= their primary's,
 change our makefile convention to be that we provide a "touch $@" action
 not an empty rule.  Also, make sure that this rule actually gets invoked
 during a distprep run, else the hazard remains.
 
 It's been like this a long time, so back-patch to all supported branches.
 
 In HEAD, I skipped the changes in src/backend/catalog/Makefile, because
 those rules are due to get replaced soon in the bootstrap data format
 patch, and there seems no need to create a merge issue for that patch.
 If for some reason we fail to land that patch in v11, we'll need to
 back-fill the changes in that one makefile from v10.
 
 Discussion: https://postgr.es/m/18556.1521668179@sss.pgh.pa.us