libsdl was built with USE=X alsa fbcon joystick opengl pulseaudio sound video xinerama xv", ldd output doesn't show libXt. Furthermore, it was built with ABI_X86=32, while libXt was -32 and it still built successfully. So, it seems either libXt is not needed at all for USE="X" or USE="X" is a to weak condition to require that dep.
Fixed. Thanks
erm, ldd is bogus on libsdl and shows almost nothing
...but what little could be read from configure.in seems to suggest libXt was yet another underlinking workaround (for libnas, in this case).
In fact I let libXt dependency for nas only