dev-perl/CryptX cbundles libtommath.
They already allow us  to unbundle it ("experimental"), but we don't yet do it in the ebuild.
(In reply to Sam James from comment #1)
> (and libtomcrypt)
tomcrypt not yet in portage.
And in general, there are often issues with using non-bundled sources. :/
Now it is. Shall we try it?
(In reply to Sam James from comment #3)
> Now it is. Shall we try it?
It doesnt build against the release version. Bundled version is development branch snapshot.
(tested with libtommath-1.2.0, libtomcrypt-1.18.2-r2, CryptX-0.072)