Thank you, Jose! With this information and some Googling I've finally
resolved the issue. For the record, here's what I had to do for the
FreeBSD package (and I assume it's similar for the other *nix's).
1) Install the x11-fonts/dejavu port. This should install the fonts
somewhere like /usr/X11R6/lib/X11/fonts/dejavu.
2) Open the file LILYPONDBASE/usr/etc/fonts/local.conf and add the
following line just after the '<fontconfig>' line. (Adjust as necessary
for your hierarchy.)
<dir>/usr/X11R6/lib/X11/fonts</dir>
All your system fonts should now be accessible!