ac: Better defaults for PAM module install path.
ac: Better defaults for PAM module install path.

On darwin (osx, macos) default to /usr/lib/pam, on linux to the
same default as before and in all other cases to /usr/lib.

Yubico/pam-u2f,Yubico/pam-u2f,Yubico/pam-u2f