Allow Rust build using locally supplied crates or crates.io

This adds a couple of configure commands to control whether we're
requiring all dependencies to be available locally (default) or not
(--enable-cargo-online-mode). When building from a tarball, we require
the RUST_DEPENDENCIES variable to point to the local repository of
crates. This also adds src/ext/rust as a git submodule that contains
such a local repository for easy setup.

1 files changed, 0 insertions, 0 deletions

diff --git a/src/ext/rust b/src/ext/rust
new file mode 160000
+Subproject 240296800824e40b10cb8c16da0e71156335394