Usually Meson uses pre-serialized information in meson-private to
speed up re-runs.  This is not possible for version changes, where
Meson instead rebuilds the serialized information using cmd_line.txt.
In some cases cmd_line.txt can contain stale information, since it
is not rebuild except when "meson setup" is invoked.  Update it in
the configure script to allow upgrading our Meson version.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>