Created attachment 147515 [details] [diff] [review] is this it? Is this what you're thinking, pch? That ifdef seems appropriate to me. Maybe I'm wrong.
I think that this dependency should rather be eliminated if possible. what header requires it?
Bug 398573 removed the |REQUIRES += toolkitcomps| line: https://hg.mozilla.org/mozilla-central/diff/cc3240bc917a/uriloader/exthandler/Makefile.in Bug 513032 removed the resulting empty conditional: https://hg.mozilla.org/mozilla-central/diff/e936fa034d28/uriloader/exthandler/Makefile.in