vim-patch:8.1.1540: cannot build without +eval #10729

Problem:    Cannot build without the +eval feature.
Solution:   Define ex_const if needed.
0abb4273f6

src/nvim/ex_docmd.c

@@ -4060,10 +4060,9 @@ static char_u *replace_makeprg(exarg_T *eap, char_u *p, char_u **cmdlinep)
   return p;
 }
 
-/*
- * Expand file name in Ex command argument.
- * Return FAIL for failure, OK otherwise.
- */
+// Expand file name in Ex command argument.
+// When an error is detected, "errormsgp" is set to a non-NULL pointer.
+// Return FAIL for failure, OK otherwise.
 int expand_filename(exarg_T *eap, char_u **cmdlinep, char_u **errormsgp)
 {
   int has_wildcards;            /* need to expand wildcards */