Windows: Update build-mingw64-toolchain.sh for modern versions.

I've ensured that MinGW-w64 toolchains can be built with the
 revisions 5166, 5445 and 5578 (at time of writing the latest).
 For recent versions, a patch is needed for widl.exe usage
 (which generates .idl files). This patch has been submitted
 to the mingw-w64-public mailing list so hopefully won't be
 needed in the future.

Change-Id: I03d5b06b4dc3055cde42efb62465440b93ff38e9
2 files changed