Cleanup dead CMake checks

Delete many unnecessary checks from CMake files. These are either completely unused or checked by something else already.

There are still more checks that create a #define which is never checked, but the checked function/header is simply used unconditionally. I did not remove any of these checks. For example, I believe the mem*, str*, and math function checks are unnecessary, but I could not find a good reference saying so.

Please check individual commit messages for why a check might be removed.