[CMAKE] Make secp256k1 build standalone

Summary:
The current cmake build doesn't allow for building secp256k1 standalone.
This diff fixes this.

Test Plan:

cd src/secp256k1
mkdir buildcmake && cd buildcmake
cmake -GNinja ..
ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3827