bzip2: fix cross build on mingw by using autoconf patch

[mcmtroffaes]

Replaces #10820. [I have a second less conservative version of the patch which I'll submit shortly.]

Main changes compared to #10820:

• use fetchurl to get the autoconf patch to avoid checking in a large file
• rebased against staging

Tested against the current nixpkgs-unstable channel (git hash 3087ef3) and then rebased against staging.

• bzip2 byte-for-byte identical output (except for hashes)
• bzip2.crossDrv byte-for-byte identical output (except for hashes) with the following shell.nix script:

import <nixpkgs>
{
  crossSystem = {
    config = "i686-w64-mingw32";
    arch = "i686";
    libc = "msvcrt";
    platform = { };
    openssl.system = "mingw";
  };
}

Not tested:

• darwin (I have no access to hardware)

cc @aszlig @peti

[mcmtroffaes]

I've now finished testing this against staging (i.e. the tree exactly as in this pull request) as well, both the native and cross builds, with the same positive results. Again, darwin wasn't tested as I don't have the hardware to do so, but given that the native build is untouched, I cannot see a reason this would cause any problems.

[mcmtroffaes]

With #11127 merged, this can be closed.