Fix fetching crates on old versions of libcurl.

[ehuss]

CURLOPT_PIPEWAIT was added in 7.43 but MacOS 10.9 has 7.30.

Fixes #6155

[alexcrichton]

Oh dear, good catch!

Currently though the performance of fetches does actually rely on PIPEWAIT because we issue all HTTP requests to libcurl simultaneously and it would otherwise flood crates.io with new TCP connections (one per crate) if PIPEWAIT weren't set.

Ideally we'd throttle sending requests to libcurl without PIPEWAIT available, but perhaps it's best to get something working before we game that out? (I think adding that will be difficult)

[ehuss]

Doesn't set_max_host_connections handle that? I ran with debugging, and it only opens 2 connections.

[alexcrichton]

@bors: r+

Aha right yes of course!

[bors]

📌 Commit 38fd476 has been approved by alexcrichton

[bors]

⌛ Testing commit 38fd476 with merge 8685531e5bf177bd63b36de92cac45e42c7f83d8...

[bors]

💔 Test failed - status-appveyor

[alexcrichton]

@bors: retry

[bors]

⌛ Testing commit 38fd476 with merge d6a4ab114d92d4fd9dfb080ed0b6dadcb319c4c5...

[bors]

💔 Test failed - status-appveyor

[ehuss]

@bors retry

[bors]

⌛ Testing commit 38fd476 with merge 34610eb...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 34610eb to master...