build: further refine build.ninja rules

In commit b0fcc6fc7fc1 ("build: rebuild build.ninja using
"meson setup --reconfigure"", 2023-05-19) I changed the build.ninja
rule in the Makefile to use "meson setup" so that the Makefile would
pick up a changed path to the meson binary.

However, there was a reason why build.ninja was rebuilt using $(NINJA)
itself.  Namely, ninja has its own cache of file modification times,
and if it does not know about the modification that was done outside
its control, it will *also* try to regenerate build.ninja.  This can be
simply by running "make" on a fresh tree immediately after "configure";
that will trigger an unnecessary meson run.

So, apply a refinement to the rule in order to cover both cases:

- track the meson binary that was used (and that is embedded in
  build.ninja's reconfigure rules); to do this, write build.ninja.stamp
  right after executing meson successfully

- if it changed, force usage of "$(MESON) setup --reconfigure" to
  update the path in the reconfigure rule

- if it didn't change, use "$(NINJA) build.ninja" just like before
  commit b0fcc6fc7fc1.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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