I've tried with USE_SYSTEM_LIBS=yes, but this decreased binary only by 1M from 34M to 33M, it is huge. The cause are fonts linked into binary, variety of fonts, including many of Eastern ones. It is very good for using without any toolkit and external fonts, but the new mupdf is huge, not a mu one, i think )
So it appears to be usefull only for minimal X neighbourhood. I think it is useless for full Xorg, especially if some toolkits are already loaded. And now i am not sure that such a huge extension with basic functional should find its place in TinyCore repo.
But there is no other FB2 viewer at all.
And TOC in .pdf's is accessible only in mupdf-gl binary, which i failed to make work at all.