NO_GEN_XPT is referenced in rules.mk but nowhere else. It doesn't appear to be used and thus I think it can be removed. https://mxr.mozilla.org/mozilla-central/search?string=NO_GEN_XPT
I have no idea what its purpose was, but it doesn't look useful.
It used to be used by the old plugins code, before Josh rewrote it.
Created attachment 645366 [details] [diff] [review] Remove NO_GEN_XPT Trivial patch.