Had a bit of a problem with re-emerging app-arch/rpm-4.4.6-r3 after an update of libexpat: the emerge failed with a whole bunch of compiler messages.
After a quick search on Gentoo Forums, I found a post about someone who has had the same problem, and was able to solve it.
About halfway the merge, the compile failed. At the beginning of a long list, I found these error messages:
Because of this error, rpm2cpio did not work because of the upgrade to libexpat-2 (which causes a whole bunch of dependency issues when not taken care of). And because of that, I was unable to install a bunch of other packages - netscape-flash was one of them.
The thread I found talked about re-emerging net-misc/neon, to which the .h files seem to belong to:
emerge -av net-misc/neon
After that was merged, the merge of app-arch/rpm-4.4.6-r3 went like a charm, and with it, I was back on track with my mega-upgrade of my workstation at work (running Gentoo, of course).