I found that support platforms and a binutils version I can check with:

#ld -V

GNU ld (GNU Binutils for Ubuntu) 2.22
  Supported emulations:
   elf_x86_64
   elf32_x86_64
   elf_i386
   i386linux
   elf_l1om
   elf_k1om

So ld -V shows that you should use in --enable-targets.

I didn't found any way for options so I tried and check and this is my current set of options:

  ./configure --prefix=/opt/dev-tools-4.9            \
--exec-prefix=/opt/dev-tools-4.9       \
 --with-sysroot=/   --enable-multilib     \
 --with-lib-path=/opt/dev-tools-4.9/lib \
 --disable-werror  --enable-shared \
 --with-pkgversion="GNU Binutils for Ubuntu" \
 --enable-targets=x86_64-linux-gnu,i686-linux-gnu,x86_64-pc-mingw32,i686-pc-mingw32
make -j $(nproc || grep -c ^processor /proc/cpuinfo|| echo "1") && \
make install

After installing your toolchain in nonstandard places you need to set environment(maybe system-wide) correctly for GCC using LIBRARY_PATH and C_INCLUDE_PATHor CPLUS_INCLUDE_PATH.

Environment Variables Affecting GCC

I see three ways to automate setting path variables for your relocatable toolchain:

• on every relocation adding your GCC path to your PATH environment variable. And create alias in your busybox profile (usually /etc/profile)

  alias example:

  alias gcc='TOOLCHAIN_PREFIX=$(which gcc | rev | cut -d"/" -f3-10 |rev); \
             LIBRARY_PATH=$TOOLCHAIN_PREFIX/lib/ \
             C_INCLUDE_PATH=$TOOLCHAIN_PREFIX/include/ gcc'
  

• creating for your toolchain launcher-script that will calculate pathes, but you'll should launch it with direct path, setting it when you launch build process, or of course you can add its location to PATH environment varaible.

  script example

  #!/bin/sh
  TOOLCHAIN_PREFIX=$(echo $0 | rev | cut -d"/" -f3-10 |rev);
  LIBRARY_PATH=$TOOLCHAIN_PREFIX/lib/ \
      C_INCLUDE_PATH=$TOOLCHAIN_PREFIX/include/ \
      $TOOLCHAIN_PREFIX/bin/gcc-4.*
  

• The most reliable and ergonomic way — create install/uninstall script that will unpack and set environment correctly, to relocate toolchain you will uninstall from it from one prefix and install to another. If you have dpkg on your debian-testing system, .deb package is best choice.

I can see no way to set environment fully automatically. But we can reduce it to setting just one path — path of toolchain.

HINT* For better stability you should isolate your toolchain and also install in your prefix Linux Kernel headers and Glib