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

 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