gcompat: provide backwards compatible functions for older gtk versions

Attempt to do something that'll work equally well across gtk versions,
to improve the supported range.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/gerror.c b/gerror.c
index c0a4679..98b51a8 100644
--- a/gerror.c
+++ b/gerror.c
@@ -40,7 +40,7 @@
 	} else {
 		char buffer[256];
 		snprintf(buffer, sizeof(buffer), "Failed to open file.");
-		gtk_label_set(GTK_LABEL(ui->error_label), buffer);
+		gtk_label_set_text(GTK_LABEL(ui->error_label), buffer);
 	}
 }