fix build error

I got build error on mingw32/windows.

https://gist.github.com/mattn/2652a97bc8c8d733e525

This mean, if compiler doesn't have c++11(or doesn't specified -std=c++11), cmath won't provide isanf/isnan to "C" namespace.

https://github.com/kazuho/picojson/blob/master/picojson.h#L168-L172

If __cplusplus less than 201103L, we should define isnan/isinf.