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/structs.h b/src/structs.h
index 1e1b1da..5745df2 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2133,6 +2133,9 @@
 #ifdef FEAT_LISP
     int		b_p_lisp;	/* 'lisp' */
 #endif
+#ifdef FEAT_MBYTE
+    char_u	*b_p_menc;	/* 'makeencoding' */
+#endif
     char_u	*b_p_mps;	/* 'matchpairs' */
     int		b_p_ml;		/* 'modeline' */
     int		b_p_ml_nobin;	/* b_p_ml saved for binary mode */