ZDoom

Hmm I don't know for sure, but my first guess would be that it could have something to do with wrong optimizations. By default, Chris' patch sets the minimum architecture to pentium and optimizes for athlon-xp. If this isn't the kind of machine you have, try changing ARCH_TYPE and TUNE_TYPE in ...

Since your compiler complains about -mtune being an invalid option, I suppose you have older version of gcc. Older versions of gcc used -mcpu, in newer versions -mcpu is deprecated (still available, but not recommended), and replaced by -mtune. You should be able to compile zdoom by changing -mtune ...