`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/


	include/ruby/missing.h: defined(__cplusplus) before using __cplusplus

	* include/ruby/missing.h (isinf, isnan): For non-C++ programs,
	  defined(__cplusplus) may be needed before using __cplusplus.
	  [Bug #14816]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@64434 b2dd03c8-39d4-4d8f-98ff-823fe69b080e