Add bundled-windows feature

[kpcyrd]

This allows a dependency to use build_bundled on windows and build_linked everywhere else. This is currently not possible because cargo doesn't support target-specific features and selecting the bundled feature for windows would enable the bundled feature everywhere.

Related: rust-lang/cargo#1197

[thomcc]

FWIW I'm pretty sure you can work around the lack of target-specific features by having a target-specific dependency on a dummy crate that does nothing but turn on the feature.

I don't oppose adding this, though.

[kpcyrd]

I've considered using a dummy crate, but figured it would be cleaner to add it to libsqlite3-sys instead, especially when packaging the resulting tree for distros.

[gwenn]

Thanks.