[SECP256K1] CMake: make the GMP bignum support optional

Summary: It is disabled by default.

Test Plan:
Run the Travis build (see
https://travis-ci.org/Fabcien/secp256k1/builds/642963633).

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5092