Log Message:
"flex" apparently changed its behavior wrt "yylineno": If invoked
w/o "-l", yylinelo still gets defined in recent versions, but not
in the version which comes with NetBSD-4.
So require "flex" for the build and always use the "-l" \ 
flag, so we'll
hopefully get deterministic behavior.
Should fix PR pkg/42509 by Robert Elz.