The PostgreSQL hackers told me not to use it for any non-core extensions, and indeed, it is not mentioned in the extension docs. The PGXN client doesn't set it, so some folks have had trouble installing twitter_fdw. So I suggest removing it, and just assuming that you're always using PGXS. Or add a NO_PGXS option if you really need a way to work around it, as I've done for pgTAP.
Right, and in fact I removed it from my kmeans module. Will do it soon, with the FDW fix for 9.2 new API.
Yet I fixed USE_PGXS option only for now. I need to look at FDW later.