Win64 fix for missing _isnanf

Per discussion in #1108, mingw64 seems to erroneously add an underscore to `isnanf` function calls.  This fix prevents a crash of `unimplemented function msvcrt.dll._isnanf` when running the 64-bit build in Wine.

This potential bug was discovered while troubleshooting a win64 crash with vesa's new `memmgr` branch slated for the 1.2 release.