BUILD: prepare the makefile to skip format lines in SUBVERS and VERDATE

We're going to put format lines in these files for use by git archive,
so let's ensure that the current default format still works. For this
we'll use two lines and only take the first one without a format tag.
BUILD: use format tags in VERDATE and SUBVERS files

The first line now contains a git format tag asking git-archive to
place the last commit's commit date and the last commit's abbreviated
ID respectively. The makefile will use these information in preference
when they're available and git is not available.

Now it's only necessary to add the two following lines in
.git/info/attributes to have the files automatically filled by git-archive :

SUBVERS export-subst
VERDATE export-subst