Add secp256k1 modules to the cmake/ninja build.

Summary: This is also adding a configure style config.h which obviate the need to carry a ton of define over compilation units.

Test Plan:

cmake -GNinja ..
ninja

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D813