Sort input files so command lines do not change. This allows Ninja to skip unnecessary build steps.

ninjabackend.py

@@ -67,9 +67,11 @@ class NinjaBuildElement():
         self.elems.append((name, elems))
 
     def write(self, outfile):
+        # Sort inputs so the command line does not change.
+        # This allows Ninja to skip unnecessary rebuilds.
         line = 'build %s: %s %s' % (' '.join([ninja_quote(i) for i in self.outfilenames]),\
                                     self.rule,
-                                    ' '.join([ninja_quote(i) for i in self.infilenames]))
+                                    ' '.join([ninja_quote(i) for i in sorted(self.infilenames)]))
         if len(self.deps) > 0:
             line += ' | ' + ' '.join([ninja_quote(x) for x in self.deps])
         if len(self.orderdeps) > 0: