Meson can't recover from deletion of generated QAPI file(s)

[Markus Armbruster]

Watch this:

    $ rm qapi/qapi-types-error.h 
    $ make
    Generating qemu-version.h with a meson_exe.py custom command
    Compiling C object qom/libqom.fa.p/object_interfaces.c.o
    In file included from ../qom/object_interfaces.c:4:
    /work/armbru/qemu/include/qapi/error.h:275:10: fatal error: 
qapi/qapi-types-error.h: No such file or directory
      275 | #include "qapi/qapi-types-error.h"
          |          ^~~~~~~~~~~~~~~~~~~~~~~~~
    compilation terminated.
    make: *** [Makefile.ninja:348: qom/libqom.fa.p/object_interfaces.c.o] Error 
1

To recover, I have to run qapi-gen.py manually and exactly right, or
blow away the build tree and start over.

The old build system did what a build system should: it remade the files
that are missing or out of date.

I'm still too clueless about Meson to debug this, but I hope I can learn
from watching you fix it.