[CMAKE] Don't use C++ features for building secp256k1

Summary:
Secp256k1 is a pure C library. As such there should be no requirement
for a C++ compiler to be able to build it.

Test Plan:
Build secp256k1 as part of bitcoin-abc and as a standalone project:

ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4853