build: rebuild build.ninja using "meson setup --reconfigure"

Do not use the rule in build.ninja, because the path to meson is hardcoded
in build.ninja and this breaks if meson moves (for example if the distro
meson suddenly becomes too old after an update).

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Makefile

@@ -115,15 +115,15 @@ Makefile.ninja: build.ninja
 	  $(NINJA) -t query build.ninja | sed -n '1,/^  input:/d; /^  outputs:/q; s/$$/ \\/p'; \
 	} > $@.tmp && mv $@.tmp $@
 -include Makefile.ninja
+endif
 
+ifneq ($(MESON),)
 # A separate rule is needed for Makefile dependencies to avoid -n
 build.ninja: build.ninja.stamp
 $(build-files):
 build.ninja.stamp: meson.stamp $(build-files)
-	$(NINJA) $(if $V,-v,) build.ninja && touch $@
-endif
+	$(MESON) setup --reconfigure $(SRC_PATH) && touch $@
 
-ifneq ($(MESON),)
 Makefile.mtest: build.ninja scripts/mtest2make.py
 	$(MESON) introspect --targets --tests --benchmarks | $(PYTHON) scripts/mtest2make.py > $@
 -include Makefile.mtest