meson incremental build doesn't handle config file going away

[Peter Maydell]

I've noticed that the meson incremental build doesn't seem to
cleanly handle a config file going away, as ppc64abi32-linux-user
has recently. The build fails with:

../../meson.build:1941:2: ERROR: Failed to load
/mnt/nvmedisk/linaro/qemu-from-laptop/qemu/configs/targets/ppc64abi32-linux-user.mak:
[Errno 2] No such file or directory:
'/mnt/nvmedisk/linaro/qemu-from-laptop/qemu/configs/targets/ppc64abi32-linux-user.mak'

where line 1941 is
  config_target += keyval.load('configs/targets' / target + '.mak')

Rerunning make doesn't help; you have to manually re-run
configure.

Something here should figure out that "config file deleted" means
it needs to rerun configure, I think.

thanks
-- PMM