ok, I finally solved the problem, it is due to a missing flag during compile. based on http://wiki.gentoo.org/wiki/Project:Emacs/GNU_Emacs_developer_guide
gconf Use gconf to read the system font name (Emacs 23 and above).
gsettings Use gsettings (glib) to read the system font name (Emacs 24 and above).
gconf Use gconf to read the system font name (Emacs 23 and above).
but they are not enabled as default compiling.
so i add
echo 'app-editors/emacs xft gsettings gconf' >> /etc/portage/package.use
and then recompile & build emacs.
finally we have
$ ldd `which emacs` | grep libXft
libXft.so.2 => /usr/lib64/libXft.so.2 (0x00007f9c1eed6000)