ninjatool: rebuild multi-output targets if outputs are missing

The "stamp file trick" used to group targets of a single multi-output rule
prevents the user from deleting one such target in order to force its
rebuild.  Doing so will not touch the stamp file, and therefore only
the dummy ":" command will be executed.

With this patch, ninjatool writes rules that force-rebuild the stamp
file if any of its outputs are missing.  Rebuilding the missing
target therefore causes the stamp file to be rebuilt too.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/ninjatool.py

@@ -908,6 +908,9 @@ def end_build(self, scope, out, iout, rule, in_, iin, orderdep):
             else:
                 stamp = '%s@%s.stamp' % (rule, sha1_text(targets)[0:11])
             self.print('%s: %s; @:' % (targets, stamp))
+            self.print('ifneq (%s, $(wildcard %s))' % (targets, targets))
+            self.print('.PHONY: %s' % (stamp, ))
+            self.print('endif')
             self.print('%s: %s | %s; ${ninja-command-restat}' % (stamp, inputs, orderonly))
             self.rule_targets[rule].append(stamp)
             self.stamp_targets[rule].append(stamp)