add run-time stringify_infnan setter method

mode=0: null
mode=1: stringify
mode=2: old unstringified nan/inf, invalid json.
See GH #32 and #11
Remove unneeded infnan prompt at build-time
fix t/117_numbers.t for cygwin and MSWin32
fix for croak format str, adjust cygwin TODO msg

centos5 with glibc 2.5 also has no -nan. treat it as cygwin's newlib.
tested ok also on openbsd 4.9, mingw, activestate perl, cygwin.
add Solaris support for inf/nan
Release 3.0112: weaken windows nan tests

activestate seems to use a different msvcrt than my mingwc msvcrt