Adjust configure to insist on Perl version >= 5.8.3.

Adjust configure to insist on Perl version >= 5.8.3.

Previously it only checked for version >= 5.8.0, although the
documentation has said that the minimum version is 5.8.3 since
commit dea6ba939.  Per the discussion leading up to that commit,
I (tgl) left it that way intentionally because you could, at the
time, do some bare-bones stuff with 5.8.0.  But we aren't actually
testing against anything older than 5.8.3, so who knows if that's
still true.  It's pretty unlikely that anyone would care anyway,
so let's just make configure's version check match the docs.

Dagfinn Ilmari Mannsåker

Discussion: https://postgr.es/m/87y278s6iq.fsf@wibble.ilmari.org
Discussion: https://postgr.es/m/16894.1501392088@sss.pgh.pa.us