Normally "prove" and "perl" come from the same Perl installation,
but we support the case where they don't (mainly because the MSys
buildfarm animals need this).  In that case, AX_PROG_PERL_MODULES
is completely the wrong thing to use, because it's checking what
"perl" has.  Instead, make a little TAP test script including the
required modules, and run that under "prove".

We don't need ax_prog_perl_modules.m4 at all after this change,
so remove it.

Back-patch to all supported branches, for the buildfarm's benefit.
(In v10, this also back-patches the effects of commit 264eb03a.)

Andrew Dunstan and Tom Lane, per an observation by Noah Misch

Discussion: https://postgr.es/m/E1moZHS-0002Cu-Ei@gemulon.postgresql.org