Make configure check for minimum required version of IPC::Run.

Make configure check for minimum required version of IPC::Run.

Per the discussion around 3eb1f4d09, let's have configure verify that
the available IPC::Run version is at least 0.79, the agreed-on minimum.
It seems unlikely that this could bite anybody anymore, but it's useful
as documentation.  (Based on that, there's little need to back-patch.)

For consistency, also supply a minimum version for the other Perl
module we have an explicit check for, Time::HiRes.  I used the
version that ships with Perl 5.8.3.

Discussion: https://postgr.es/m/E1mYY6Z-0006OL-QN@gemulon.postgresql.org