Builds have been broken since reworking autogeneration in c618f91e and
related, this gets fixed here by applying similar qstr generation logic
for the msvc builds: c files are only preprocessed when changed (or not
yet preprocessed) and the concatenated output is fed into makeqstrdefs.py.
To speed up this process, the concatenated output is already filtered to
contain only lines which makeqstrdefs really needs: this makes the qstr
generation stage about twice as fast (checked on different machines).