Missing external dependencies on Windows: libsodium and gnugpg

[rylev]

When trying to run the top 100 "crypto" crates, there are two crates that fail to build due to external dependencies, rust_sodium-sys and gpgme-sys which need libsodium and gnupg respectively.

We should consider adding these to the build environment for Windows. Both are made available on Linux.

[ecstatic-morse]

libsodium was added in #11. GPGME isn't available via vcpkg, and doesn't appear to support Windows anyway.

[ecstatic-morse]

Looks like the build script for rust_sodium-sys needs RUST_SODIUM_LIB_DIR to be set on Windows, otherwise it will try to download the library over the network.