When I was trying to emerge Inkscape 0.38.1 it did'nt list libsigc++ as a requirement but when it was going through ./configure it was asking for it. So I installed it seperetly and Inkscape installed fine after that. Reproducible: Always Steps to Reproduce: 1. 2. 3.
It should dependend on any: libsigc++ of the slot 1.2 (I have it already installed from the slot 1.3: 2.0.1 and it still does not work). After installing the 1.2.5 version from the slot 1.2 the inkscape installation directly works.
I changed the dependency so that libsigc++-1.2.* will be used. Thanks for the bug-report!