Autodetect availability of c extension

This improves default behavior of setup.py in sense that py version of
frozendict is used for python versions which do not have C extension
available yet. This would probably reduce amount of issues similar to
[1-4] in future.

[1] Marco-Sulla#68
[2] Marco-Sulla#67
[3] Marco-Sulla#65
[4] Marco-Sulla#58

setup.py

@@ -164,7 +164,7 @@
 impl = python_implementation()
 
 if custom_arg == None:
-    if impl == "PyPy":
+    if impl == "PyPy" or not src_path.exists():
         custom_arg = "py"
     else:
         custom_arg = "c"