Fix configure for LLVM 3.8

[dongz9]

Fix the following error when running configure with llvm 3.8svn:

configure: error: bad LLVM version: 3.8.0svn, need >=3.5

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @huonw (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 The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

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[steveklabnik]

@bors: r+

[bors]

📌 Commit d77a249 has been approved by steveklabnik

[eddyb]

@bors r+

[bors]

📌 Commit 0ad631c has been approved by eddyb

[bors]

⌛ Testing commit 0ad631c with merge 79c6a4d...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt