[secp256k1] Fix the Cirrus build

Summary: The last commit descriptor has been backported without the required adaptation to our codebase, so let's fix it. Note the build will only work with autotools as it uses a hack to override the compiler at build time.

Test Plan: Read it and compare to other builds.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16984