Fix #if-skipping to *really* skip the skipped group.

Previously we were avoiding printing within a skipped group, but we
were still evluating directives such as #define and #undef and still
emitting diagnostics for things such as macro calls with the wrong
number of arguments.

Add a test for this and fix it with a high-priority rule in the lexer
that consumes the skipped content.

glcpp.h

@@ -127,6 +127,7 @@ struct glcpp_parser {
 	yyscan_t scanner;
 	struct hash_table *defines;
 	string_list_t *active;
+	int lexing_if;
 	int space_tokens;
 	int newline_as_space;
 	int in_control_line;