What's the current theory about derived files in VPATH builds?

It seems like VPATH building is broken at the moment, at least if you
are working from a maintainer-clean source tree.

make -C bootstrap all
make[3]: Entering directory `/home/postgres/build/src/backend/bootstrap'
/usr/local/bin/bison  -o bootparse.c /home/postgres/pgsql/src/backend/bootstrap/bootparse.y
/usr/local/bin/flex  -o'bootscanner.c' /home/postgres/pgsql/src/backend/bootstrap/bootscanner.l
gcc -O1 -Wall -Wmissing-prototypes -Wpointer-arith -fno-strict-aliasing -g -I/home/postgres/pgsql/src/backend/bootstrap
-I../../../src/include-I/home/postgres/pgsql/src/include -D_XOPEN_SOURCE_EXTENDED   -c -o bootparse.o bootparse.c
 
/home/postgres/pgsql/src/backend/bootstrap/bootparse.y:401: bootscanner.c: No such file or directory
make[3]: *** [bootparse.o] Error 1

I thought the current plan was that derived files should be generated in
the build tree (as indeed it seems to be doing) ... but there are
apparently parts of the Makefiles that are not in sync with this plan.
Has this just not been tested recently, or am I confused?
        regards, tom lane