vim-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)

2c7292dc5b

src/nvim/option_defs.h

@@ -489,6 +489,7 @@ EXTERN char_u   *p_lcs;         // 'listchars'
 EXTERN int p_lz;                // 'lazyredraw'
 EXTERN int p_lpl;               // 'loadplugins'
 EXTERN int p_magic;             // 'magic'
+EXTERN char_u   *p_menc;        // 'makeencoding'
 EXTERN char_u   *p_mef;         // 'makeef'
 EXTERN char_u   *p_mp;          // 'makeprg'
 EXTERN char_u   *p_cc;          // 'colorcolumn'
@@ -736,6 +737,7 @@ enum {
   , BV_KP
   , BV_LISP
   , BV_LW
+  , BV_MENC
   , BV_MA
   , BV_ML
   , BV_MOD