Link SHA-512 code when using -DED25519_SHA512

Link SHA-512 code when using -DED25519_SHA512

When the $CFLAGS variable contains the -DED25519_SHA512 option (by
default it doesn't), the code from src/optional/sha512.c is
automatically linked to the final libraries (libmonocypher.a and
libmonocypher.so).

That way, users who need to install a ED25519 compliant version of
Monocypher can do so simply by altering the compilation options with the
$CFLAGS variable.