PAM preferes the crypt_gensalt_r function over its internal
crypt_make_salt function, when this function is provided by
the system's crypt library.
libxcrypt now ships (and used to ship it until v3.1.1) such an
alias for its crypt_gensalt_rn function, which features the
same semantics and the same prototype as the crypt_gensalt_r
function existing on some systems.
