Add support for 64-bit Windows to build.rs

Fixes linker issues on 64-bit Windows.

pkg-config doesn't support 64-bit Windows as a target, so build.rs silently returns, leaving us without the build flags we need to link everything.

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)).
This won't work if you're cross compiling. You'll want to examine the `TARGET` environment variable instead.