Work around MSVC's deficient <cmath>

Change libsample/transform.cpp to use the MSVC-specific _finite on
Windows, rather than std::isfinite, which MSVC does not implement.

Change-Id: If7b33e17bcb7c2e4432d670d44b559205f88796d
Reviewed-by: John Ehresman <jpe@wingware.com>