pgsql: Fix platform and Perl-version dependencies in new jsonb_plperl c

Fix platform and Perl-version dependencies in new jsonb_plperl code.

Testing SvTYPE() directly is more fraught with problems than one might
think, because depending on context Perl might be storing a scalar value
in one of several forms, eg both numeric and string values. This resulted
in Perl-version-dependent buildfarm test failures. Instead use the SvTYPE
test only to distinguish non-scalar cases (AV, HV, NULL). Disambiguate
scalars by testing SvIOK, SvNOK, then SvPOK. This creates a preference
order for how we will resolve cases where the value is available in more
than one form, which seems fine to me.

Furthermore, because we're now dealing directly with a "double" value
in the SvNOK case, we can get rid of an inadequate and unportable
string-comparison test for infinities, and use isinf() instead.
(We do need some additional #include and "-lm" infrastructure to use
that in a contrib module, per prior experiences.)

In passing, prevent the regression test results from depending on DROP
CASCADE order; I've not seen that malfunction, but it's trouble waiting
to happen.

Discussion: https://postgr.es/m/E1f3MMJ-0006bf-B0@gemulon.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/331b2369c0ad1e51d5e50bf5dd75232e0160553a

Modified Files
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contrib/jsonb_plperl/Makefile | 2 +
contrib/jsonb_plperl/expected/jsonb_plperl.out | 26 ++++++---
contrib/jsonb_plperl/expected/jsonb_plperlu.out | 26 ++++++---
contrib/jsonb_plperl/jsonb_plperl.c | 76 ++++++++++++++-----------
contrib/jsonb_plperl/sql/jsonb_plperl.sql | 16 +++++-
contrib/jsonb_plperl/sql/jsonb_plperlu.sql | 16 +++++-
6 files changed, 110 insertions(+), 52 deletions(-)