_project.py: Include fail-on-overlap setting in cache key

Fixes #473.

buildstream/_project.py

@@ -164,8 +164,12 @@ class Project():
         if self._cache_key is None:
 
             # Anything that alters the build goes into the unique key
-            # (currently nothing here)
-            self._cache_key = _cachekey.generate_key({})
+            cache_key_dict = {}
+
+            if self.fail_on_overlap:
+                cache_key_dict['fail-on-overlap'] = True
+
+            self._cache_key = _cachekey.generate_key(cache_key_dict)
 
         return self._cache_key