Configure for isfinitel / isinfl.
Perl_isfinite() enhancements.

Use isfinitel() if available, prefer C99 isfinite() over BSD finite().
OSX does have isfinite(), and 10.9+ prefers it over finite().

isfinite() is a macro+inline while Configure tests it as a library symbol.
Perl_isinf enhancements.

Try isfinitel first, and require Perl_isnan.
Use Perl_isfinite().

Also kill obsolete/wrong comment: Perl_frexp is not casting its
argument to NV, and if USE_LONG_DOUBLE Perl_frexp is frexpl.
And if we do have long double and don't have frexpl, it is unclear
what should we do? (in this particular case, casting to double might
be the best possible thing to do anyway.)
Win32 apparently has _isnan() and _finite(), and _fpclass().

(At least, if one believes the Oracle of Google.)
Perl_signbit should return non-zero for -0.
Separate grok_infnan() from grok_number().

Remaining issues:

(1) would need tests, but there are two problems: [a] generating inf/nan
    reliably and testing for it from Perl level is hard (see items (2) and
    (3) below), and [b] the behavior of various systems with especially NaN
    differs (some platforms might throw SIGFPEs).

(2) toke.c:scan_number() will not call this code (via grok_number)
    because "NaN" or "Inf" do not look at all like floats to it.

(3) Even as we now recognize these forms, the native strtod()
    might not (problem of cross-portability of these exceptional
    forms: Win32 outputs e.g. "1.#INF", what Linux reading this should do,
    or conversely Linux outputs "Inf", what should Win32 do?)