Building GCC 4.9 biarch (i686+x86_64) on Debian 6.0 i686

[Alex Blekhman <a.tinsmith@xxxxxxxxx>]

Dear group,

I have difficulties building GCC 4.9.2 on Debian 6 i686.

My goal is to be able to compile both i686 and x86_64 binaries on the 
same Debian 6.0 i686 machine. Default compilation target should be i686, 
and x86_64 should be available via switch -m64.

Here is what I did:

$ ls
binutils-2.25          <-- latest binutil sources
binutils-2.25-build    <-- empty dir
gcc-4.9.2              <-- gcc-4.9.2 sources
gcc-4.9.2-build        <-- empty dir

$ mkdir /opt/gcc-4.9
$ export PATH=/opt/gcc-4.9:$PATH

$ cd binutils-2.25-build
$ ../binutils-2.25/configure --prefix=/opt/gcc-4.9/ --target=i686-elf --
enable-multilib --enable-targets=i686-elf,x86_64-elf
$ make
$ make install

$ cd ../gcc-4.9.2
$ ./contrib/download_prerequisites

$ cd ../gcc-4.9.2-build
$ ../gcc-4.9.2/configure                    \
    --prefix=/opt/dolby/gcc-4.9             \
    --enable-languages=c,c++                \
    --program-suffix=-4.9                   \
    --with-sysroot                          \
    --enable-shared                         \
    --enable-threads=posix                  \
    --enable-clocale=gnu                    \
    --with-system-zlib                      \
    --enable-targets=i686-elf,x86_64-elf    \
    --enable-multiarch                      \
    --with-arch-32=i686                     \
    --with-multilib-list=m32,m64,mx32       \
    --enable-multilib                       \
    --with-tune=generic                     \
    --enable-checking=release               \
    --build=i686-elf                        \
    --host=i686-elf                         \
    --target=i686-elf
$ make

The build fails with the error:

The directory that should contain system headers does not exist:
  /opt/gcc-4.9/i686-elf/sys-root/usr/include

I tried various guides, but none of them gives simple step-by-step 
instruction how to do it.

Any help is greatly appreciated.

Regards
Alex