the latest sigil ebuild is for version 0.6.2 from 2012.12.06, more than two years old. there are 10 newer versions.
Sigil 0.8.4 is out since 2015.02.01. i don't know how difficult it would be to update the ebuilds, but now that qt5 is supported perhaps it would be possible?
I have a working ebuild for sigil-0.8.2, but 0.8.4 adds a dependency on, among other things, Qt5 Web Channels  that gentoo doesn't seem to have an ebuild for yet.
Version 0.8.2 added dependencies on >=qt-5.1.0, linguist-tools, qtconcurrent and cmake-3.
The other added dependencies since 0.8.2 are for >=qt-5.4.0, dev-qt/qtpositioning:5 and dev-qt/qtsensors:5 but these we do have in the tree. I haven't looked at the code to see what the last two are used for, though.
I'll upload the ebuilds I've used here if someone wants to take it from there.
Created attachment 398086 [details]
Provisional ebuild for Sigil 0.8.4
Created attachment 398088 [details]
Diff from the 0.6.2 ebuild
Created attachment 398090 [details]
A minimal ebuild for dev-qt/qtwebchannel-5.4.1:5
Dependencies are based on the corresponding arch linux package.
*** This bug has been marked as a duplicate of bug 465436 ***
(In reply to Jouni Kosonen from comment #2)
> Provisional ebuild for Sigil 0.8.4
thanks! the ebuild failed a couple of times during configuration due to some missing dependencies. i got it working adding these lines in CDEPEND:
i guess the ebuild needs some more testing, though. anyway, i got sigil working now. thanks again four your prompt solution.