isnan() function is evidently not present on <math.h> on Windows
before Visual Studio 2013. We define it on win32_port.h using
_isnan(). However _isnan() is also not present. It is on <float.h>.
The patch is attached to include this from win32_port.h.
Thanks to Thomas Munro for point this out to me [1]. It is hard to
notice this issue without testing the changes on Windows.
[1] https://www.postgresql.org/message-id/CAEepm%3D3dE0w%3D_dOcELpPum6suze2NZwc%3DAV4T_xYrDUoHbZJvA%40mail.gmail.com