How to allow .cpp suffix for gbuild ?

Hi,

I've done 
http://cgit.freedesktop.org/libreoffice/core/commit/?id=38cc4f7fdee8f673a2b97ff7665941f83d3775a2
because in boost, there are ~4000 .cpp files and we need to use 4 of them.
So I thought it's silly to rename them just because we use .cxx suffix.

But my commit does not seem to work reliably.
http://tinderbox.libreoffice.org/cgi-bin/gunzip.cgi?tree=MASTER&brief-log=1346617801.8957
Caolán had to do make clean in his box I guess ?
And now I am trying to use _set_cxx_suffix more widely and am getting
the same problems.

So, do you have any ideas, how to allow cpp suffix ?
Maybe at least for now, I could add gb_LinkTarget_add_external_exception_object
and assume that suffix for external sources is .cpp ?

The main problem is that we depend on generated source file in gbuild
and I don't know how to make that dependency with cxx-suffix variable.

Thanks,
Matus