Repeated build failures in expr due to yyparse

Running a make -s -j5 buildworld;  cleaned the entire obj tree meanwhile.
Builds are running on a slightly older HEAD thought with an updated libc
and some headers.

I can reproduce this all the time.  Buildworld at this stage should be
self-contained, right?  So why is it failing?

===> bin/expr (all)
cc1: warnings being treated as errors
/zoo/bz/HEAD.svn/bin/expr/expr.y: In function 'main':
/zoo/bz/HEAD.svn/bin/expr/expr.y:291: warning: implicit declaration of function 'yyparse'
/zoo/bz/HEAD.svn/bin/expr/expr.y:291: warning: nested extern declaration of 'yyparse'
expr.c: At top level:
expr.c:813: warning: no previous prototype for 'yyparse'
*** Error code 1


-- 
Bjoern A. Zeeb                                 You have to have visions!
   It does not matter how good you are. It matters what good you do!