Redmine #5874: Do not compile users promises if pw functions not found

This is needed for NetBSD, which has PAM, but not fgetpwent and
friends.
Redmine #5874: Add --without-pam option to turn off compiling with PAM
Merge pull request #1768 from kacfengine/without-pam_option

Redmine #5874: Some users promise compile fixes