glcpp: Add boolean 'error' flag.

We used to check if the info log is non-empty, but when we print
warnings, this will no longer be valid.
diff --git a/glcpp/glcpp.h b/glcpp/glcpp.h
index e67469b..8167a48 100644
--- a/glcpp/glcpp.h
+++ b/glcpp/glcpp.h
@@ -155,6 +155,7 @@
 	token_node_t *lex_from_node;
 	char *output;
 	char *info_log;
+	int error;
 };
 
 glcpp_parser_t *