Support bitrig in autoconf build system.

[dhuseby]

Patch by Dave Huseby.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230669 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 298988e)

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[dhuseby]

@alexcrichton this is the cherry picked llvm revision to add bitrig support. with this in, and once I get my snapshot on the snapshot server, then my bitrig build slave should be able to build rust bitrig automatically.

[alexcrichton]

Pushed to the rust-llvm-2015-02-19 branch, thanks!

[dhuseby]

@alexcrichton will you bump the submodule revision in rust to point to this new revision?

[alexcrichton]

Yeah that's all that's left to be done, do you want to make the PR? (we'd run the update through bors as usual)

[dhuseby]

sure I'll do it.