Ok, you all can stop ranting about ebuilds etc. being broken.
I found the culprit, it seems something wrong in the kdelibs building.
Basically, when kdelibs builds dcopidl, it happily *says* "oh, this
would need libXft, that in turn has a broken link to libexpat.so.0.
But who cares, let's go along with it!" instead of stopping and
telling me that libXft is broken.
I have no more the relevant compilation output snippet, but that's the
meaning. So, I just emerged libXft, fixing the expat link, and
everything went pretty smooth.
Now I have the problem that Thunderbird starts, but no window comes
out, but it's a different stuff...
firstname.lastname@example.org mailing list