I do not see my CXXFLAGS into the virtualbox build. If you think that they could break, feel free to add the custom-cflags IUSE.
It does not respect the CXXFLAGS partially. If you can't reproduce by yourself I will post a log.
Please attach the log. Even if this is true debugging this problem might become quite cumbersome as VB's build system (kbuild) IMHO is a mess. Also my experience with VB upstream regarding kbuild problems is not the best (see https://bugs.gentoo.org/352322#c3). So finding a fix or getting a fix from upstream might take very long.
Created attachment 361532 [details] build log Portage 2.2.7 (default/linux/amd64/13.0, gcc-4.7.3, glibc-2.15-r3, 3.4.65-gentoo x86_64) ================================================================= System uname: Linux-3.4.65-gentoo-x86_64-Intel-R-_Core-TM-2_Duo_CPU_E8400_@_3.00GHz-with-gentoo-2.2 KiB Mem: 8114348 total, 5967712 free KiB Swap: 8388604 total, 8388604 free Timestamp of tree: Mon, 21 Oct 2013 07:15:01 +0000 ld GNU ld (GNU Binutils) 2.23.1 app-shells/bash: 4.2_p45 dev-lang/python: 2.7.5-r3 dev-util/cmake: 2.8.10.2-r2 dev-util/pkgconfig: 0.28 sys-apps/baselayout: 2.2 sys-apps/openrc: 0.11.8 sys-apps/sandbox: 2.6-r1 sys-devel/autoconf: 2.13, 2.69 sys-devel/automake: 1.12.6, 1.13.4 sys-devel/binutils: 2.23.1 sys-devel/gcc: 4.7.3-r1 sys-devel/gcc-config: 1.7.3 sys-devel/libtool: 2.4.2 sys-devel/make: 3.82-r4 sys-kernel/linux-headers: 3.9 (virtual/os-headers) sys-libs/glibc: 2.15-r3 Repositories: gentoo x-portage ACCEPT_KEYWORDS="amd64" ACCEPT_LICENSE="*" CBUILD="x86_64-pc-linux-gnu" CFLAGS="-march=core2 -O2" CHOST="x86_64-pc-linux-gnu" CONFIG_PROTECT="/etc /usr/share/config /usr/share/gnupg/qualified.txt /usr/share/themes/oxygen-gtk/gtk-2.0 /usr/share/themes/oxygen-gtk/gtk-3.0" CONFIG_PROTECT_MASK="/etc/ca-certificates.conf /etc/env.d /etc/fonts/fonts.conf /etc/gconf /etc/gentoo-release /etc/revdep-rebuild /etc/sandbox.d /etc/terminfo" CXXFLAGS="-march=core2 -O2" DISTDIR="/usr/portage/distfiles" EMERGE_DEFAULT_OPTS="--with-bdeps y" FCFLAGS="-O2 -pipe" FEATURES="assume-digests binpkg-logs collision-protect config-protect-if-modified distlocks ebuild-locks fixlafiles merge-sync multilib-strict news parallel-fetch preserve-libs protect-owned sandbox sfperms strict unknown-features-warn unmerge-logs unmerge-orphans userfetch userpriv usersandbox usersync" FFLAGS="-O2 -pipe" GENTOO_MIRRORS="http://distfiles.gentoo.org http://gentoo.wheel.sk/ http://mirror.netcologne.de/gentoo/ http://mirrors.linuxant.fr/distfiles.gentoo.org/" LANG="it_IT.UTF-8" LDFLAGS="-Wl,-O1 -Wl,--as-needed -Wl,--hash-style=gnu" MAKEOPTS="-j2" PKGDIR="/usr/portage/packages" PORTAGE_CONFIGROOT="/" PORTAGE_RSYNC_OPTS="--recursive --links --safe-links --perms --times --omit-dir-times --compress --force --whole-file --delete --stats --human-readable --timeout=180 --exclude=/distfiles --exclude=/local --exclude=/packages" PORTAGE_TMPDIR="/tmp/" PORTDIR="/usr/portage" PORTDIR_OVERLAY="/usr/local/portage" SYNC="rsync://rsync.de.gentoo.org/gentoo-portage" USE="X acl alsa amd64 berkdb bzip2 cli consolekit cracklib crypt cups cxx dbus dri fortran gdbm gif iconv jpeg kde mmx modules mp3 mudflap multilib mysql ncurses nptl ogg opengl openmp pam pcre png policykit qt3support qt4 readline session sse sse2 ssl tcpd threads tiff udev unicode vorbis zlib" ABI_X86="64" ALSA_CARDS="ali5451 als4000 atiixp atiixp-modem bt87x ca0106 cmipci emu10k1x ens1370 ens1371 es1938 es1968 fm801 hda-intel intel8x0 intel8x0m maestro3 trident usb-audio via82xx via82xx-modem ymfpci" ELIBC="glibc" INPUT_DEVICES="keyboard mouse evdev" KERNEL="linux" LIBREOFFICE_EXTENSIONS="presenter-console presenter-minimizer" LINGUAS="en en_GB" OFFICE_IMPLEMENTATION="libreoffice" PHP_TARGETS="php5-5" PYTHON_SINGLE_TARGET="python2_7" PYTHON_TARGETS="python2_7" RUBY_TARGETS="ruby19 ruby18" USERLAND="GNU" VIDEO_CARDS="radeon r600" USE_PYTHON="2.7"
Please reopen if this is still an issue.