I'm not sure if this is the best way to accomplish this, but it does look like it works so it seems fine.

WWW: http://monkey.org/~jose/software/libdistance

Yeah, it's supposed to respect MANPREFIX and MAN[section]PREFIX. It works on a test using default value for these variables

To be honest, I was at a bit of a loss here as to the correct way to do things. I had kind of expected variables like PREFIX, LIBDIR, etc., to Just Work (tm), but the default behaviour of bsd.lib.mk seems to be staging things in usr/lib rather than usr/local/lib.

Is there a better way for me to direct the library, include files, etc., to the correct place?

Here's another instance of me adding PREFIX and LIBDIR variables, which feels like it shouldn't be necessary.

You can start reading /usr/ports/Mk/bsd.port.mk header comments, there you can find all basic variables used by ports tree

So, I read bsd.port.mk, as well as bsd.lib.mk, and the problem is that LIBDIR doesn't respect PREFIX (or LOCALBASE) by default: the default is to hard-code it to /usr/lib (see /usr/share/mk/bsd.own.mk:170). So, while I am able to drop the PREFIX?= from my patches, I think that I still need to set LIBDIR, etc., or else I get errors like:

===>   Generating temporary packing list
install -C -o root -g wheel -m 444   libdistance.a /usr/home/jon/freebsd-ports/devel/libdistance/work/stage/usr/lib/
install: /usr/home/jon/freebsd-ports/devel/libdistance/work/stage/usr/lib/: No such file or directory
*** Error code 71

A (hopefully) final question: if I wanted to install this binary in ${PREFIX}/tests/libdistance instead of ${PREFIX}/tests, is there a way to have the ports system automagically do the mkdir, or do I need to do it manually?

I'd be tempted to add

MAKE_ARGS+= LIBDIR=${PREFIX}/lib

in the port Makefile but given the need to patch it's probably a wash.

I think you'll just have to do the mkdir. The bsd.*.mk bits assume mtree has created directories so they don't include a mechanism.