Replace bashisms with POSIX syntax

[jeroen-mostert]

This replaces some bashisms (notably equality check with == and substring replacement) with POSIX equivalents, for distros where sh operates in strict POSIX mode. I will not claim that all bashisms are thereby purged from the configure files (they may lurk in conditionals not taken), but at least it allows a basic build to proceed.

[swx-jenkins3]

Can one of the admins verify this patch?

[manjugv]

@jeroen-mostert What is your affiliation? Thanks.

[jeroen-mostert]

This patch is a product of my spare time, neither commissioned by, nor approved of, nor sponsored by, nor produced during working hours, nor aligned with the purposes of the company I also happen to be employed for. I will not sign any document, waiver, contract or agreement to release any rights associated with it, but release this code into the public domain as per the CC0 universal deed, including but not limited to giving others permission to use the same code for merging purposes without attribution. I hope that answers your question.