Cargo.toml: Make some more packages optional on Windows
Cargo.toml: Make some more packages optional on Windows

The underlying issue is that these packages' build systems may not have
pkg-config files generated for us for their Visual Studio build system,
so we feed things into the linker instead.

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/565>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg