glcpp: Output to a buffer and error log rather than directly printing.

In the standalone case, simply print the buffers when done.
diff --git a/glcpp/glcpp.h b/glcpp/glcpp.h
index 4459daa..ac103b7 100644
--- a/glcpp/glcpp.h
+++ b/glcpp/glcpp.h
@@ -141,6 +141,8 @@
 	skip_node_t *skip_stack;
 	token_list_t *lex_from_list;
 	token_node_t *lex_from_node;
+	char *output;
+	char *errors;
 };
 
 glcpp_parser_t *