isnan
=====

POSIX specification:
 https://pubs.opengroup.org/onlinepubs/9699919799/functions/isnan.html

Gnulib module: isnan

Portability problems fixed by Gnulib:

  * This macro is missing on some platforms: MSVC 9.

  * isnan was introduced with C99 and is thus commonly not present on pre-C99 systems.

  * isnan is not a macro on some platforms: IRIX 6.5, Solaris 11.4.

  * This macro does not work on some platforms: MSVC 14.

  * On IRIX 6.5 with cc, isnan does not recognize some NaNs.

Portability problems not fixed by Gnulib:

  * This macro returns an unspecified result when given noncanonical values such as unnormalized numbers, pseudo-denormals, pseudo-NaNs, pseudo-Infinities, and pseudo-zeroes.