Upon starting x loading r600_dri.so fails

first bad commit :
http://cgit.freedesktop.org/mesa/mesa/commit/?id=f70c3853513637fa6ed38e75f73d472a9fa61213

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  ./autogen.sh --prefix=/usr \
    --sysconfdir=/etc \
    --with-dri-driverdir=/usr/lib/xorg/modules/dri \
    --with-gallium-drivers=r300,r600,radeonsi,swrast \
    --with-dri-drivers=swrast \
    --enable-gallium-llvm \
    --enable-egl \
    --enable-gallium-egl \
    --with-egl-platforms=x11,drm,wayland \
    --enable-shared-glapi \
    --enable-gbm \
    --enable-glx-tls \
    --enable-dri \
    --enable-glx \
    --enable-osmesa \
    --enable-gles1 \
    --enable-gles2 \
    --enable-texture-float \
    --enable-xa \
    --enable-vdpau \
    --enable-gallium-gbm \
    --enable-gallium-g3dvl \
    --enable-xvmc \
    --enable-xorg \
    --enable-r600-llvm-compiler \
    --with-llvm-shared-libs

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I've chosen blocker as severity, as this commit is a candidate for the 9.1
branch