Use bash in configure

[steveklabnik]

Many *NIXes alias /bin/sh to bash, but not all do, and this is
a bash script, not an sh script.

r? @brson

[tshepang]

Does the script fail with dash?

[sfackler]

It fails with Solaris's sh, at the very least: #25845

[globin]

Please use /usr/bin/env bash. The path to bash is not necessarily deterministic.

[sfackler]

Is the path to env more deterministic than the path to bash?

[globin]

http://en.wikipedia.org/wiki/Shebang_%28Unix%29#Portability

I am mostly using NixOS which only has /bin/sh and /usr/bin/env in the normal locations and everything else gets added to my PATH dynamically depending on my current environment I'm using. Because of that I can have multiple versions of software/libraries installed without them interfering.
E.g. that means no need for multirust.

[steveklabnik]

Good call @globin

[brson]

This has always been intended to be a sh script, not a bash script, as are all of our shell scripts. There are unix environments without bash - consider busybox. I have actually put a great deal of effort to avoid bashisms, and I'm not yet feeling the motivation to abandon that. So I'm not convinced.

[brson]

I want to keep aiming for sh compatibility for the time being.