[SECP256K1] CMake: add an option to enable endomorphism

Summary:
Default to OFF. This is equivalent to the autotools
--enable-endomorphism configure option.

Test Plan:
Run the Travis build (see
https://travis-ci.org/Fabcien/secp256k1/builds/642867595).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5089