coredata: Fix changing prefix option

This is a regression from #11537.

mesonbuild/coredata.py

@@ -837,10 +837,10 @@ class CoreData:
         pfk = OptionKey('prefix')
         if pfk in options:
             prefix = self.sanitize_prefix(options[pfk])
-            self.options[OptionKey('prefix')].set_value(prefix)
+            dirty |= self.options[OptionKey('prefix')].set_value(prefix)
             for key in BULITIN_DIR_NOPREFIX_OPTIONS:
                 if key not in options:
-                    self.options[key].set_value(BUILTIN_OPTIONS[key].prefixed_default(key, prefix))
+                    dirty |= self.options[key].set_value(BUILTIN_OPTIONS[key].prefixed_default(key, prefix))
 
         unknown_options: T.List[OptionKey] = []
         for k, v in options.items():

unittests/allplatformstests.py

@@ -4601,21 +4601,21 @@ class AllPlatformTests(BasePlatformTests):
         olddata = filename.read_bytes()
         oldmtime = os.path.getmtime(filename)
 
-        for opt in ('-Dstring=val', '--buildtype=plain', '-Dfeature=disabled'):
+        for opt in ('-Dstring=val', '--buildtype=plain', '-Dfeature=disabled', '-Dprefix=/abc'):
             self.setconf([opt])
             newdata = filename.read_bytes()
             newmtime = os.path.getmtime(filename)
-            self.assertEqual(olddata, newdata)
             self.assertEqual(oldmtime, newmtime)
+            self.assertEqual(olddata, newdata)
             olddata = newdata
             oldmtime = newmtime
 
-        for opt in ('-Dstring=abc', '--buildtype=release', '-Dfeature=enabled'):
+        for opt in ('-Dstring=abc', '--buildtype=release', '-Dfeature=enabled', '-Dprefix=/def'):
             self.setconf([opt])
             newdata = filename.read_bytes()
             newmtime = os.path.getmtime(filename)
-            self.assertNotEqual(olddata, newdata)
             self.assertGreater(newmtime, oldmtime)
+            self.assertNotEqual(olddata, newdata)
             olddata = newdata
             oldmtime = newmtime