[SECP256K1] Travis: pass extra flags to the CMake build

Summary:
Improve the global environment variables layout, rename EXTRAFLAGS to
AUTOTOOLS_EXTRA_FLAGS and set the CMake equivalent
CMAKE_EXTRA_FLAGS.

Depends on D5092.

Test Plan:
Run the Travis build (see
https://travis-ci.org/Fabcien/secp256k1/builds/643101709).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5094