bin/178819: bmake w/ WRKDIRPEFIX=/tmp breaks Ports Collection

[Alexey Dokuchaev]

Yes, I've seen the fresh import; will test it tomorrow.  I'm not sure what
is the problem, but "ports should be able to control the value of MAKEFILE
as desired" looks strange.  MAKEFILE is one of the standard bsd.port.mk's
variables, and ports were always able to control it.  Are you referring to
something else (e.g. internal bmake(1)'s variable)?  And how does this all
got affected by WRKDIRPREFIX=/tmp?