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/runtime/doc/quickref.txt b/runtime/doc/quickref.txt
index b55fce1..e64a5f6 100644
--- a/runtime/doc/quickref.txt
+++ b/runtime/doc/quickref.txt
@@ -782,6 +782,7 @@
 'macatsui'		    Mac GUI: use ATSUI text drawing
 'magic'			    changes special characters in search patterns
 'makeef'	  'mef'     name of the errorfile for ":make"
+'makeencoding'	  'menc'    encoding of external make/grep commands
 'makeprg'	  'mp'	    program to use for the ":make" command
 'matchpairs'	  'mps'     pairs of characters that "%" can match
 'matchtime'	  'mat'     tenths of a second to show matching paren