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)

src/structs.h

@@ -2132,6 +2132,9 @@ struct file_buffer
     char_u	*b_p_kp;	/* 'keywordprg' */
 #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' */