better performance in find_firsts

_is_build_cache not set in dictionary storage

test/test_cache.py

@@ -18,6 +18,13 @@ def make_description():
     return OptionDescription('od1', '', [u1, u2, u3])
 
 
+def test_cache_config():
+    od1 = make_description()
+    assert od1.impl_already_build_caches() == False
+    c = Config(od1)
+    assert od1.impl_already_build_caches() == True
+
+
 def test_cache():
     od1 = make_description()
     c = Config(od1)