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

contrib/jsonb_plperl/sql/jsonb_plperl.sql

@@ -34,14 +34,27 @@ $$;
 SELECT testSVToJsonb();
 
 
+-- unsupported (for now)
 CREATE FUNCTION testRegexpToJsonb() RETURNS jsonb
 LANGUAGE plperl
 TRANSFORM FOR TYPE jsonb
 AS $$
+my $a = qr/foo/;
+return ($a);
+$$;
+
+SELECT testRegexpToJsonb();
+
+
+-- this revealed a bug in the original implementation
+CREATE FUNCTION testRegexpResultToJsonb() RETURNS jsonb
+LANGUAGE plperl
+TRANSFORM FOR TYPE jsonb
+AS $$
 return ('1' =~ m(0\t2));
 $$;
 
-SELECT testRegexpToJsonb();
+SELECT testRegexpResultToJsonb();
 
 
 CREATE FUNCTION roundtrip(val jsonb) RETURNS jsonb
@@ -83,4 +96,5 @@ SELECT roundtrip('{"1": "string1"}');
 SELECT roundtrip('{"1": {"2": [3, 4, 5]}, "2": 3}');
 
 
+\set VERBOSITY terse \\ -- suppress cascade details
 DROP EXTENSION plperl CASCADE;