Portability fix for the configure scripts generated

Summary:
Found thanks to the developer checks from the pkgsrc software
distribution (for NetBSD, SmartOS, Minix, MacOS X, Linux, and more).

This is a backport of secp256k1's PR511

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4978