It seems that a few packages (example: sysutils/findutils) want to use
nroff during installation, probably to generate man pages. On systems
that come with a native nroff this is not a issue, of course.
On IRIX 5.3 however, which SGI shipped without these text tools (I think
they used to sell them separately!) this fails.
One way to solve this is to simply add
USE_TOOLS+= nroff
(hey, the buildlink framework has grown really powerful over the last
year!). Strictly speaking I think on can say that this line is simply
missing in that Makefile. If I understand correctly how buildlink works
it should not have any impact on systems that already come with a native
nroff, does it?
Do you think it is OK to add that line?

-- 
Georg Schwarz    http://home.pages.de/~schwarz/
 georg.schwarz@freenet.de  +49 178 8545053