Add support for 64-bit Windows to build.rs

Fixes linker issues on 64-bit Windows.

The 64-bit libraries from [here](http://slproweb.com/products/Win32OpenSSL.html) do not provide MinGW .a files, only VC files, so these must be converted before they can be used (see [here](https://github.com/sfackler/rust-openssl/issues/111)).