`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

include/ruby/missing.h

@@ -168,6 +168,8 @@ RUBY_EXTERN const union bytesequence4_or_float rb_nan;
 #    include <ieeefp.h>
 #    endif
 #  define isinf(x) (!finite(x) && !isnan(x))
+#  elif __cplusplus >= 201103L
+#    include <cmath> // it must include constexpr bool isinf(double);
 #  else
 RUBY_EXTERN int isinf(double);
 #  endif
@@ -176,7 +178,11 @@ RUBY_EXTERN int isinf(double);
 
 #ifndef isnan
 # ifndef HAVE_ISNAN
+#  if __cplusplus >= 201103L
+#    include <cmath> // it must include constexpr bool isnan(double);
+#  else
 RUBY_EXTERN int isnan(double);
+#  endif
 # endif
 #endif