This amends commit f8be71ac.

This still should resolve FS-4303.

What's going on here is that we need a portable way to access
strftime.  date(1posix) doesn't provide enough.  And without perl, I
can't think of a better way to get to it than just using C.  So the
logic for generating the extended revision has been moved into a small
self-contained and hopefully portable C program.