Comment 6 for bug 1227956

I'm quite sure that the problem is the same as with cmake and autotools: In build.xml a static library is built out of all the inkscape files:
> <target name="lib" depends="compile"
> description="create a static library">
> <staticlib command="${archutil}ar crsv"
> file="${build}/libinkscape.a">

And the linker doesn't even include the code to register "svg:svg". Search the binary for it, you probably won't find its registration.
So the solution to this problem is to adjust build.xml to directly link together all the files and not building the static library at all.