From: tim1948 on
The directory containing the sources is supposed to be specified by
the -m switch if it is not /usr/src. But is it?

$ cd /usr/src
$ ls
8 cddl release
COPYRIGHT contrib rescue
LOCKS crypto sbin

This is part of it: it is recursive in the sense that, if I switch to
8 (the sources that were downloaded and used for the buildworld
procedure) its directory structure looks the same as the original. So
is it -m /usr/src/8? Or do they assume that since most users probably
do it that way, to avoid overwriting other sources in that directory,
this is the default location?