Make PrBoomX Compatible with Complevel 21 (MBF21)

[PorpiangWriter]

I know that DSDA-Doom and Woof! have it, however, I like the original PrBoom+ so much to suggest this 'faithful' fork to be compatible with the newer MBF21 standard.

[JadingTsunami]

I've considered it. I might block demo-compatibility if I do implement it. I'd have to think about it.