[SECP256K1] Fix travis failure on ECMULT_GEN_PRECISION

Summary:
The auto value is invalid on cmake, this case should be handled to not
assign any value to SECP256K1_ECMULT_GEN_PRECISION so that the default
behavior is tested.

Test Plan:
Run the Travis build (see
https://travis-ci.org/Fabcien/secp256k1/builds/657636884, failures are
unrelated and deserve a separate fix).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5407