patch 8.0.1295: cannot automatically get a server name in a terminal

Problem:    Cannot automatically get a server name in a terminal.
Solution:   Add the --enable-autoservername flag to configure. (Cimbali,
            closes #2317)
diff --git a/src/feature.h b/src/feature.h
index e90d755..d100d6b 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -1168,6 +1168,14 @@
 #endif
 
 /*
+ * +autoservername	Automatically generate a servername for clientserver
+ *			when --servername is not passed on the command line.
+ */
+#if defined(FEAT_CLIENTSERVER) && !defined(FEAT_AUTOSERVERNAME)
+/* # define FEAT_AUTOSERVERNAME */
+#endif
+
+/*
  * +termresponse	send t_RV to obtain terminal response.  Used for xterm
  *			to check if mouse dragging can be used and if term
  *			codes can be obtained.