pgsql: Probe $PROVE not $PERL while checking for modules needed by TAP

Probe $PROVE not $PERL while checking for modules needed by TAP tests.

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 264eb03aa.)

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

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

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e9af18c6c5512566c409c2ff99083fc1d89ff314

Modified Files
--------------
aclocal.m4                     |   1 -
config/ax_prog_perl_modules.m4 |  77 -----------------------------
config/check_modules.pl        |  20 ++++++++
configure                      | 108 +++++++----------------------------------
configure.ac                   |  26 +++++-----
5 files changed, 53 insertions(+), 179 deletions(-)