CLEANUP: 51d: use the build options list to report it

This removes one #ifdef from haproxy.c.
diff --git a/src/51d.c b/src/51d.c
index 74b7928..a44b40b 100644
--- a/src/51d.c
+++ b/src/51d.c
@@ -648,4 +648,5 @@
 	sample_register_fetches(&sample_fetch_keywords);
 	sample_register_convs(&conv_kws);
 	cfg_register_keywords(&_51dcfg_kws);
+	hap_register_build_opts("Built with 51Degrees support.", 0);
 }