Step 1:
	Re-install one system with USE="unicode" ONLY.
Before completing USE after first boot:

USE="doc" emerge libusb 
[ebuild  N    ] app-text/sgml-common-0.6.3-r4  
[ebuild  N    ] app-text/opensp-1.5.1  USE="nls" 
[ebuild  N    ] app-text/openjade-1.3.2-r1  
[ebuild  N    ] app-arch/unzip-5.52-r1  
[ebuild  N    ] app-text/docbook-sgml-dtd-4.2-r2  
[ebuild   R   ] dev-libs/libusb-0.1.10a  USE="doc*"

At this point, libusb failed 

Step 2: base on list of docbook installed in past (see first message)
	use ebuild for one,test libusb, if failed -> ebuild unmerge, retry with next
		app-text/docbook-sgml-dtd-4.1-r3  
		app-text/docbook-sgml-dtd-4.0-r3  
		app-text/docbook-dsssl-stylesheets-1.79  
		app-text/docbook-sgml-dtd-3.0-r3  
		app-text/docbook-sgml-dtd-3.1-r3  
		app-text/docbook-xml-simple-dtd-4.1.2.4-r2  
		app-text/docbook-xml-simple-dtd-1.0-r1  
		app-text/build-docbook-catalog-1.2  
		app-text/docbook-xsl-stylesheets-1.68.1-r1  
		app-text/docbook-xml-dtd-4.4-r1  
		app-text/docbook-sgml-utils-0.6.14  USE="-tetex" 
		app-text/docbook-sgml-dtd-4.4  

Multiple error on ebuild and test BUT with docbook-sgml-utils 

ebuild  /usr/portage/app-text/docbook-sgml-dtd/docbook-sgml-utils-0.6.14.ebuild merge
Failed -> use emerge -p
[ebuild  NS   ] app-text/docbook-sgml-dtd-4.1-r3  
[ebuild  NS   ] app-text/docbook-sgml-dtd-4.0-r3  
[ebuild  N    ] dev-perl/SGMLSpm-1.03-r5  
[ebuild  N    ] app-text/docbook-dsssl-stylesheets-1.79  
[ebuild  NS   ] app-text/docbook-sgml-dtd-3.0-r3  
[ebuild  NS   ] app-text/docbook-sgml-dtd-3.1-r3  
[ebuild  N    ] www-client/lynx-2.8.5-r3  USE="ipv6 nls ssl -bzip2 -cjk" 
[ebuild  N    ] app-text/docbook-xml-simple-dtd-4.1.2.4-r2  
[ebuild  N    ] dev-libs/libxml2-2.6.26  USE="ipv6 python readline -debug -doc -test" 
[ebuild  N    ] app-text/docbook-xml-simple-dtd-1.0-r1  
[ebuild  N    ] app-text/build-docbook-catalog-1.2  
[ebuild  N    ] app-text/docbook-xsl-stylesheets-1.68.1-r1  
[ebuild  N    ] app-text/docbook-xml-dtd-4.4-r1  
[ebuild  N    ] app-text/docbook-sgml-utils-0.6.14  USE="-tetex" 

emerge docbook-sgml-utils
USE="doc" emerge libusb
LIBUSB OK

SOLUTION : ADD DEP IN EBUILD TO DOCBOOK-SGML-UTILS (SPEED SOLUTION!!!) NOT LONG TERM PROBABLY
----------------------------------------------------------------------------------------------

Gentoo Base System version 1.12.4
Portage 2.1-r2 (default-linux/amd64/2006.1, gcc-4.1.1, glibc-2.4-r3, 2.6.17-gentoo-r7 x86_64)
=================================================================
System uname: 2.6.17-gentoo-r7 x86_64 AMD Turion(tm) 64 Mobile Technology ML-32
app-admin/eselect-compiler: [Not Present]
dev-lang/python:     2.4.3-r1
dev-python/pycrypto: 2.0.1-r5
dev-util/ccache:     [Not Present]
dev-util/confcache:  [Not Present]
sys-apps/sandbox:    1.2.17
sys-devel/autoconf:  2.13, 2.59-r7
sys-devel/automake:  1.4_p6, 1.5, 1.6.3, 1.7.9-r1, 1.8.5-r3, 1.9.6-r2
sys-devel/binutils:  2.16.1-r3
sys-devel/gcc-config: 1.3.13-r3
sys-devel/libtool:   1.5.22
virtual/os-headers:  2.6.11-r2
ACCEPT_KEYWORDS="amd64"
AUTOCLEAN="yes"
CBUILD="x86_64-pc-linux-gnu"
CFLAGS="-O2 -mtune=k8 -pipe -fomit-frame-pointer"
CHOST="x86_64-pc-linux-gnu"
CONFIG_PROTECT="/etc"
CONFIG_PROTECT_MASK="/etc/env.d /etc/gconf /etc/revdep-rebuild /etc/terminfo"
CXXFLAGS="-O2 -mtune=k8 -pipe -fomit-frame-pointer"
DISTDIR="/usr/portage/distfiles"
FEATURES="autoconfig distlocks metadata-transfer sandbox sfperms strict"
GENTOO_MIRRORS="ftp://ftp.belnet.be/mirror/rsync.gentoo.org/gentoo/"
LINGUAS="fr en"
MAKEOPTS="-j2"
PKGDIR="/usr/portage/packages"
PORTAGE_RSYNC_OPTS="--recursive --links --safe-links --perms --times --compress --force --whole-file --delete --delete-after --stats --timeout=180 --exclude='/distfiles' --exclude='/local' --exclude='/packages'"
PORTAGE_TMPDIR="/var/tmp"
PORTDIR="/usr/portage"
SYNC="rsync://rsync.be.gentoo.org/gentoo-portage"
USE="amd64 berkdb bitmap-fonts cli crypt cups dlloader dri fortran gdbm gpm ipv6 isdnlog libg++ ncurses nls nptl nptlonly pam pcre perl ppds pppd python readline reflection session spl ssl tcpd truetype-fonts type1-fonts udev unicode xorg zlib elibc_glibc input_devices_mouse input_devices_synaptics input_devices_keyboard input_devices_joystick kernel_linux linguas_fr linguas_en userland_GNU video_cards_sis"
Unset:  CTARGET, EMERGE_DEFAULT_OPTS, INSTALL_MASK, LANG, LC_ALL, LDFLAGS, PORTAGE_RSYNC_EXTRA_OPTS, PORTDIR_OVERLAY