Fix POSIX shell in configure script (support Illumos & Solaris)

[nbaksalyar]

This patch provides the same fix that is applied for the configure script in rustc:
https://github.com/rust-lang/rust/blob/master/configure#L3-L9. It is required because #!/bin/sh is not Bash/POSIX-compatible on Solaris and its derivatives by default.

This change was previously discussed there: rust-lang/rust#28589.

[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. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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 the contribution instructions for more information.

[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit 1e38c58 has been approved by alexcrichton

[bors]

⌛ Testing commit 1e38c58 with merge c7ffb3e...

[bors]

☀️ Test successful - cargo-cross-linux, cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-gnu-32, cargo-win-gnu-64, cargo-win-msvc-32, cargo-win-msvc-64
Approved by: alexcrichton
Pushing c7ffb3e to master...