Compiling with FROZEN_MANIFEST fails to build mpy-cross if it hasn't been built separately first.

Reproducer

(on current master)

git clean -fdx && echo "" > manifest.py && make -C ports/unix submodules && make -C ports/unix FROZEN_MANIFEST="$PWD/manifest.py"

Fails linking with:

LINK build/mpy-cross
/usr/bin/ld: build/py/qstr.o: in function `qstr_init':
qstr.c:(.text.qstr_init+0x7): undefined reference to `mp_qstr_frozen_const_pool'
collect2: error: ld returned 1 exit status

Bisect

Bisect points to f01d5fb657159f1afa9d41cf39830d17b791a67e as the culprit.

I assume the problem is that the mpy-cross Makefile inherits the wrong CFLAGS? Also, interestingly cmake-based ports have the same problem.