Avoid re-expanding a macro name that has once been rejected from expansion.

The specification of the preprocessor in C99 says that when we see a
macro name that we are already expanding that we refuse to expand it
now, (which we've done for a while), but also that we refuse to ever
expand it later if seen in other contexts at which it would be
legitimate to expand.

We add a test case for that here, and fix it to work. The fix takes
advantage of a new token_t value for tokens and argument words along
with the recently added IDENTIFIER_FINALIZED token type which
instructs the parser to not even look for another expansion.

glcpp.h

@@ -42,6 +42,11 @@ typedef struct string_list {
 	string_node_t *tail;
 } string_list_t;
 
+typedef struct token {
+	int type;
+	char *value;
+} token_t;
+
 typedef struct token_node {
 	int type;
 	const char *value;
@@ -68,6 +73,7 @@ typedef struct glcpp_parser glcpp_parser_t;
 typedef enum {
 	TOKEN_CLASS_ARGUMENT,
 	TOKEN_CLASS_IDENTIFIER,
+	TOKEN_CLASS_IDENTIFIER_FINALIZED,
 	TOKEN_CLASS_FUNC_MACRO,
 	TOKEN_CLASS_OBJ_MACRO
 } token_class_t;