ARGH!  Many things (lex, vgrind, gnu/cc, et al.) are not building because,
   say, it can't figure out how to build "cc.0" from "gcc.1", or "lex.0" from
   "flex.1", et al.

You need to remove your .depend files and reinstall /usr/share/mk, to
deal with the new way man pages are generated.  This was changed, and
announced, a couple of weeks ago.

   Also, at least a few people have mentioned "make world".  Presumably,
   there's supposed to be a /usr/src/Makefile, then?

There *is* a /usr/src/Makefile.