`int isnan(double)` is a POSIXism

- `isnan` is something relatively new.  We need to provide one for
  those systems without it.  However:
- X/Open defines `int isnan(double)`. Note the `int`.
- C99 defines `isnan(x)` to be a macro.
- C++11 nukes them all, undefines all the "masking macro"s, and
  defines its own `bool isnan(double)`.  Note the `bool`.
- In C++, `int isnan(double)` and `bool isnan(double)` are
  incompatible.
- So the mess.

[Bug #14816][ruby-core:87364]
further reading: https://developers.redhat.com/blog/2016/02/29/why-cstdlib-is-more-complicated-than-you-might-think/


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63571 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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