Most of these warnings are due to the use of NULL instead of 0 or NULL instead of '\0'.
Some are macro redefinitions. None of them are pointer storage related. Those will be
in another PR.
Linux-specific byteswap.h just includes BYTESWAP.H, so there no need platform-specific header.
Fixes "non-portable path to file" ([-Wnonportable-include-path]) warnings.