While installing gentoo from scratch and trying to update with USE changed (without anything of X installed), I realized that dev-libs/opensc does not compile due to missing dependencies. Those dependencies are: x11-libs/libX11 x11-libs/libXt With those emerged manually the program now compiles without issues. Reproducible: Always
Only x11-libs/libXt if USE nsplugin
added x11-libs/libXt as dependency to opensc-0.11.[34]. Thanks David and Alon.