Decode message only if encoding is known

When python does not know the encoding of stdout, sys.stdout.encoding
is None.  Then calling decode(None) raises an exception.  We just
skip decoding when the encoding is unknown.

setup.py

@@ -265,6 +265,7 @@ def enqueue_output(out, queue):
 def _log_buf(buf):
     if not buf:
         return
+    if sys.stdout.encoding:
         buf = buf.decode(sys.stdout.encoding)
     buf = buf.rstrip()
     lines = buf.splitlines()