Visual Studio 2015 has additional warnings around noexcept and
disabling exceptions, which can be worked around with the
(undocumented) _HAS_EXCEPTIONS macro.
Visual Studio 2013 and 2015 have roundf in math.h, so use it to
avoid extra work and casts.
We avoid using cmath, as it undefs isfinite on gcc, but Visual Studio
2015 no longer provides overloads of copysign from math.h (which is
actually correct). As a result, use copysignf (which is available in
math.h in 2013 and 2015) directly.
Review URL: https://codereview.chromium.org/1244173005
b7069e9c1f