patch 8.0.0420: text garbled when the system encoding differs from 'encoding'

Problem:    When running :make the output may be in the system encoding,
            different from 'encoding'.
Solution:   Add the 'makeencoding' option. (Ken Takata)
diff --git a/src/buffer.c b/src/buffer.c
index 6edb48a..c25cb2f 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2255,6 +2255,9 @@
     clear_string_option(&buf->b_p_lw);
 #endif
     clear_string_option(&buf->b_p_bkc);
+#ifdef FEAT_MBYTE
+    clear_string_option(&buf->b_p_menc);
+#endif
 }
 
 /*