Added ENABLE_FOPEN define, if not defined, defaults on. But allows you to exclude FILE funcs

[SoylentGraham]

I was using this for a UWP app/library in unity, and UWP/win10 sdk doesn't allow fopen usage (or maybe the unsafe version, I can't remember, I was in a rush, either way, I had to remove these :)

So I needed to make a change to cleanly exclude that support.

This is that change!