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/configure.ac b/src/configure.ac
index e287124..7277f87 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -2066,6 +2066,15 @@
   AC_SUBST(TERM_OBJ)
 fi
 
+AC_MSG_CHECKING(--enable-autoservername argument)
+AC_ARG_ENABLE(autoservername,
+	[  --enable-autoservername Automatically define servername at vim startup.], ,
+	[enable_autoservername="no"])
+AC_MSG_RESULT($enable_autoservername)
+if test "$enable_autoservername" = "yes"; then
+  AC_DEFINE(FEAT_AUTOSERVERNAME)
+fi
+
 AC_MSG_CHECKING(--enable-multibyte argument)
 AC_ARG_ENABLE(multibyte,
 	[  --enable-multibyte      Include multibyte editing support.], ,