Auto merge of #3050 - nbaksalyar:configure-fix, r=alexcrichton
Auto merge of #3050 - nbaksalyar:configure-fix, r=alexcrichton

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

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: https://github.com/rust-lang/rust/pull/28589.

vojtechkral/cargo,tsurai/cargo,eminence/cargo,rust-lang/cargo,vojtechkral/cargo,rustyhorde/cargo,vojtechkral/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo,nipunn1313/cargo,sbeckeriv/cargo,rustyhorde/cargo,AndiDog/cargo,achanda/cargo,eminence/cargo,rust-lang/cargo,nipunn1313/cargo,achanda/cargo,achanda/cargo,rust-lang/cargo,eminence/cargo,tsurai/cargo,durka/cargo,achanda/cargo,tshepang/cargo,durka/cargo,rust-lang/cargo,sbeckeriv/cargo,tshepang/cargo,tsurai/cargo,AndiDog/cargo,durka/cargo,AndiDog/cargo,AndiDog/cargo,durka/cargo,sbeckeriv/cargo,tshepang/cargo,nipunn1313/cargo,sbeckeriv/cargo,AndiDog/cargo,tsurai/cargo,rustyhorde/cargo,tshepang/cargo,tsurai/cargo,vojtechkral/cargo