crc32() clashes with zlib function of the same name

[M.E. O'Neill]

OpenSSH defines a function crc32(), and so does the zlib library.  This
is at best confusing (since they are different functions with different
prototypes), and at worst a source of crashes.

I found this problem getting OpenSSH up and running on Darwin, which
turned out to be calling the wrong function.   My bandaid was to include
``-Dcrc32=crcsum32'' in CFLAGS.  The proper fix would be to either use
zlib's crc32 function if it is essentially the same as OpenSSH's or
rename OpenSSH's function if it differs.

Best regards,

    M.E.O.