NetBSD-5 has bswap32 and bswap64 as system supplied macros,
and builtin gcc is old and no builtin_bswap32 and builtin_bswap64.

Then failed to build in cipher/bithelp.h, because it defines static bswap32 and
bswap64 functions but its name will be overwritten with system macros.

Is it possible to change using system bswap32 and bswap64 if available,
or prevent to define such functions if system macros are defined?