qt-gui depending apps like vlc or mumble emerge successfully, but are killed by SIGILL. Here's why:

The qmake totally ignores your CFLAGS and runs an "autodetection":

se4_2 auto-detection... ()
x86_64-pc-linux-gnu-g++ -c -pipe -msse4.2 -march=amdfam10 -mtune=amdfam10 -m3dnow -msse2 -msse3 -msse4a -mabm -mmmx -O2 -pipe -flto=4 -fgraphite-identity -std=c+0x -O2 -Wall -W -I../../../mkspecs/linux-g+ -I. -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/freetype2 -o sse4_2.o sse4_2.cpp
x86_64-pc-linux-gnu-g++ -flto=4 -Wl,-O1 -o sse4_2 sse4_2.o -L/usr/lib64 -lgthread-2.0 -lrt -lglib-2.0 -msse4.2
sse4_2 enabled.
avx auto-detection... ()
x86_64-pc-linux-gnu-g++ -c -pipe -mavx -march=amdfam10 -mtune=amdfam10 -m3dnow -msse2 -msse3 -msse4a -mabm -mmmx -O2 -pipe -flto=4 -fgraphite-identity -std=c+0x -O2 -Wall -W -I../../../mkspecs/linux-g+ -I. -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/freetype2 -o avx.o avx.cpp
x86_64-pc-linux-gnu-g++ -flto=4 -Wl,-O1 -o avx avx.o -L/usr/lib64 -lgthread-2.0 -lrt -lglib-2.0 -mavx
avx enabled.

My CPU does NOT support AVX. But qmake enables it and qt-apps fail to run.

what did i do to resolve this?

my /usr/portage/eclass/qt4-build.eclass line 450:
conf+=" ${myconf} -no-sse4.1 -no-sse4.2 -no-avx -no-ssse3"

reemerged qt-gui, mumble and vlc, now they are working again.