Use POSIX-friendly syntax

[concatime]

Hi, with this patch, I was able to compile Kremlin on a POSIX-friendly environment, which means:

1. replace which by the POSIX command -v
2. replace bash by POSIX sh

This helps a little bit more the build process by removing two dependencies.

[ghost]

All CLA requirements met.

[msprotz]

Thanks for your contribution! Could you tell us a little bit why this is important? I buy the changes related to which, but why get rid of bash? This complicates the behavior in that we can't really rely on bash-like wildcard expansion which in turn means that, if we want to "do things right", we have to use $(wildcard ) a lot in Makefiles. Also, if you're building kremlin, surely you must have bash installed somewhere, no?

[concatime]

On my local build, bash was not required, so I wrongly thought that I could simply swap it. That being said, from the CI (even tho I cannot access the log because it requries that I log-in with a Microsoft account, seriously?), it seems important. So yeah, you're right. I restored the SHELL to bash, and will install it on my machine.