[libretro] The core treats Fennel "case" keyword as global var

When running the following code through Retroarch

```
(fn _G.TIC []
	(cls)
	(print (case (% (math.floor (/ (time) 1000)) 2)
		0 "Zero"
		1 "One")))
```

it fails with the error

```
[INFO] [Environ] SET_MESSAGE: game:3: attempt to call a nil value (global 'case')
stack traceback:
	game:3: in function 'TIC'
```

Web and Standalone work OK.