setup.py is wrong for BSD (non-Mac, Linux, Windows) machines

[jmgurney]

setup.py has an else clause for unknown operating systems:
https://github.com/flamewow/argon2_py/blob/master/setup.py#L36

This else clause causes an error:

[...]
$ python setup.py build
cc -pthread -shared -L/usr/local/lib -fstack-protector-strong build/temp.freebsd-14.0-CURRENT-arm64-3.9/./phc-winner-argon2/src/argon2.o build/temp.freebsd-14.0-CURRENT-arm64-3.9/./phc-winner-argon2/src/blake2/blake2b.o build/temp.freebsd-14.0-CURRENT-arm64-3.9/./phc-winner-argon2/src/core.o build/temp.freebsd-14.0-CURRENT-arm64-3.9/./phc-winner-argon2/src/encoding.o build/temp.freebsd-14.0-CURRENT-arm64-3.9/./phc-winner-argon2/src/ref.o build/temp.freebsd-14.0-CURRENT-arm64-3.9/./phc-winner-argon2/src/thread.o build/temp.freebsd-14.0-CURRENT-arm64-3.9/./src/argon2_py.o -L/usr/local/lib -lc_argon2 -o build/lib.freebsd-14.0-CURRENT-arm64-3.9/_argon2.cpython-39.so
ld: error: unable to find library -lc_argon2
cc: error: linker command failed with exit code 1 (use -v to see invocation)
error: command '/usr/bin/cc' failed with exit code 1

Simply converting the previous elif (https://github.com/flamewow/argon2_py/blob/master/setup.py#L32) to an else, and eliminating the clause fixes things.

Also, the define HAVE_SYSCTL_HW_USERMEM appears to not be used anywhere in the code.