POSIX specification:
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/fenv.h.html
Portability problems fixed by Gnulib:
Portability problems not fixed by Gnulib:
FE_UPWARD
is 0x100 and FE_DOWNWARD
is 0x200,
whereas in MSVC 14.30, it’s the opposite.