* Future-proof autoconf's Perl version test.

* Future-proof autoconf's Perl version test.

  GHC only works with Perl versions 5.6 and later; however
  the current test greps in a rather non-future-proof way
  (explicitly against 5.6 and 5.8).

  We instead exploit the "v-string" change in 5.6, which
  displays the version string as "v5.x.y", rather than
  the earlier format "version 5.xxx_yyy".  Hence grepping
  against "v5" will make it future proof for a while.
  (At least until Perl 6 is installed as the system perl.)