You can clone with
HTTPS or Subversion.
pam_usb is included in most Linux distributions out there.
If you happen to run Ubuntu or Debian, run:
# apt-get install libpam-usb pamusb-tools
Gentoo (You may need to unmask sys-auth/pam_usb or use --automask-write):
# emerge pam_usb
Before proceeding, make sure all dependencies are installed: libxml2, pam, udisks and pmount are installed
# make install