include: Use constants for merge mode prefixes

This will make their semantics explicit.

src/xkbcomp/ast-build.c

@@ -511,7 +511,7 @@ IncludeCreate(struct xkb_context *ctx, char *str, enum merge_mode merge)
         incl->modifier = extra_data;
         incl->next_incl = NULL;
 
-        if (nextop == '|')
+        if (nextop == MERGE_AUGMENT_PREFIX)
             merge = MERGE_AUGMENT;
         else
             merge = MERGE_OVERRIDE;

src/xkbcomp/include.c

@@ -166,7 +166,7 @@ ParseIncludeMap(char **str_inout, char **file_rtrn, char **map_rtrn,
     /* Set up the next file for the next call, if any. */
     if (*nextop_rtrn == '\0')
         *str_inout = NULL;
-    else if (*nextop_rtrn == '|' || *nextop_rtrn == '+')
+    else if (is_merge_mode_prefix(*nextop_rtrn))
         *str_inout = next;
     else
         return false;

src/xkbcomp/include.h

@@ -30,6 +30,12 @@
 /* Reasonable threshold, with plenty of margin for keymaps in the wild */
 #define INCLUDE_MAX_DEPTH 15
 
+#define MERGE_OVERRIDE_PREFIX '+'
+#define MERGE_AUGMENT_PREFIX  '|'
+#define MERGE_DEFAULT_PREFIX  MERGE_OVERRIDE_PREFIX
+#define is_merge_mode_prefix(ch) \
+    ((ch) == MERGE_OVERRIDE_PREFIX || (ch) == MERGE_AUGMENT_PREFIX)
+
 bool
 ParseIncludeMap(char **str_inout, char **file_rtrn, char **map_rtrn,
                 char *nextop_rtrn, char **extra_data);

src/xkbcomp/rules.c

@@ -810,7 +810,8 @@ append_expanded_kccgst_value(struct matcher *m, struct scanner *s,
         pfx = sfx = 0;
 
         /* Check for prefix. */
-        if (str[i] == '(' || str[i] == '+' || str[i] == '|' ||
+        if (str[i] == '(' ||
+            is_merge_mode_prefix(str[i]) ||
             str[i] == '_' || str[i] == '-') {
             pfx = str[i];
             if (str[i] == '(') sfx = ')';
@@ -895,9 +896,9 @@ append_expanded_kccgst_value(struct matcher *m, struct scanner *s,
      */
 
     ch = (darray_empty(expanded) ? '\0' : darray_item(expanded, 0));
-    expanded_plus = (ch == '+' || ch == '|');
+    expanded_plus = is_merge_mode_prefix(ch);
     ch = (darray_empty(*to) ? '\0' : darray_item(*to, 0));
-    to_plus = (ch == '+' || ch == '|');
+    to_plus = is_merge_mode_prefix(ch);
 
     if (expanded_plus || darray_empty(*to))
         darray_appends_nullterminate(*to, expanded.item, expanded.size);