MarcusDe,

Since this is the error:
    error: ‘be64toh’ was not declared in this scope
you need to make sure be64toh() is defined.

You can run "grep -r be64toh /usr/include/" to see what header file defines be64toh and add it to the top of sha512.c or sha612.h.

For example, my machine shows "/usr/include/endian.h", so I would add
   #include <endian.h>
to the top of sha512.c.

The solution/problem could be a little different, but missing the correct include is one of the more common reasons for this error.