How to activate DEBUG in Data.HashTable?

Hello, with a large Agda development, I have a reproducible segmentation fault that I have been able to localise to the serialisation (Agda.TypeChecking.Serialise.encode), which heavily relies on Data.HashTable. Now I find that Data.HashTable (from GHC-7.0.1) has a CPP-enabled DEBUG version --- is there a ``proper'' way to activate this? Possibly even so that it can be added ex-post to an existing installation? (So I don't have to re-install all packages.) mk/ways.mk says that the ``debug'' way only affects the RTS... Wolfram