cp -a --update

[John W. Eaton]

A recent change you made introduced cp -a --update to the src and
scripts Makefiles.  I don't think these options to cp are portable, so
I checked in the following change.

  http://hg.savannah.gnu.org/hgweb/octave/rev/a7e288044ef7

I think the new set of tests combined with cp and touch have the
equivalent effect, but should be portable.  Do you see any problem
with this solution?

Thanks,

jwe