Note: it is always better to use external libargon2 library.
Unfortunately, until Argon2 is in generic crypto libraries,
we must sometimes use bundled version just for bureaucratic reasons.
Let's include optimized variant of reference implementation as well.
Note, this code will not add any SSE compiler switches.
If --enable-internal-sse-argon2 option is used, it checks if current
compilation flags support simple SSE progam and if so, it use
the optimized variant.
(Not tested for AVX optimizations; it expects that SSE is enabled as well.)
