Allow usage of upstream libuv

[fabiand]

Currently libuv is a submodule in the source tree of rust. It would be nice if an external libuv could be used - and configured at build time. This would allow an easier packaging.

[lucab]

I was wondering on this topic too, but to the best of my knowledge libuv doesn't currently offer a stable API/ABI and decoupling it now may result in a useless compatibility hell. See joyent/libuv#354.
Not directly related, but just for reference this report is on the same path as llvm-unbundling (#4259).

[bstrie]

Libuv is now producing actual stable releases, so perhaps now this can move ahead.

[lucab]

I think that #6567 is temporarily blocking this.

[toddaaro]

This is certainly something that should happen eventually. Added a few tags and nominating for "production ready".

[graydon]

just a bug, removing milestone/nomination.

[flaper87]

Triage bump. Updated issue title to mention upstream instead of external. Nothing else to add

[alexcrichton]

There is currently only one more commit that we need to upstream (rust-lang-deprecated/libuv@800b56f), opened with libuv as joyent/libuv#1214