backend/ninja: don't rewrite the pickle data if it hasn't changed

Which prevents spurious rebuilds of dyndeps

mesonbuild/backend/ninjabackend.py

@@ -1103,6 +1103,14 @@ class NinjaBackend(backends.Backend):
 
         scaninfo = TargetDependencyScannerInfo(
             self.get_target_private_dir(target), source2object, scan_sources)
+
+        write = True
+        if os.path.exists(pickle_abs):
+            with open(pickle_abs, 'rb') as p:
+                old = pickle.load(p)
+            write = old != scaninfo
+
+        if write:
             with open(pickle_abs, 'wb') as p:
                 pickle.dump(scaninfo, p)