How to enable GCC i686-w64-mingw32 multilib

I always opt i686-w64-mingw32 multilib when building GCC. That mean 32bit by default and 64bit via -m64. in GCC 4.9 (or since GCC 4.8 SEH introduction) this configuration seems to be broken / unsupported. By common sense if x86_64 multilib is supported then the inverse should be possible too.

Here is the patch:

--- D:/Tarball/gcc49s/libgcc/config.host.bak Thu Mar 27 22:40:31 2014
+++ D:/Tarball/gcc49s/libgcc/config.host Sun Sep 28 14:24:20 2014
@@ -646,8 +646,20 @@
  if test x$enable_sjlj_exceptions = xyes; then
   tmake_eh_file="i386/t-sjlj-eh"
  else
-  tmake_eh_file="i386/t-dw2-eh"
-  md_unwind_header=i386/w32-unwind.h
+  case ${host} in
+    i[34567]86-w64-mingw*)
+      if [ "${enable_multilib}" = yes ]; then
+        tmake_eh_file="i386/t-seh-eh"
+      else
+        tmake_eh_file="i386/t-dw2-eh"
+        md_unwind_header=i386/w32-unwind.h
+      fi
+      ;;
+    i[34567]86-pc-mingw*)
+      tmake_eh_file="i386/t-dw2-eh"
+      md_unwind_header=i386/w32-unwind.h
+      ;;
+  esac
  fi
  # Shared libgcc DLL install dir depends on cross/native build.
  if test x${build} = x${host} ; then

Might be a bit ugly but still make sense and works! Overall that's just a naming/convention mess. With the patch we should have SJLJ in 32bit and SEH exception in 64bit mode by default (a.k.a no exception handling specified at configure).