by Download xtrlock source code from the Debian project
(I used http://ftp.debian.org/debian/pool/main/x/xtrlock/)
xmkmf
make
make install
Then, take the first of the two gcc commands that were run by make,
and rerun it manually, but add -DSHADOW_PWD to the defines section
of the command line (-Dthis -Dthat -DSHADOW_PWD).
(I tried to muck around with the 1000+ line Makefile but found it easier
to just re-run the gcc commands)
Then re-run the rest of the commands from that previous run of make install
make install.man # to install the man page
chmod u+s /usr/bin/xtrlock (if you want non-root users to use it) —
if you forget this step, it will not be able to read the /etc/shadow file
and will complain that the password entry is missing the password!