Respect LDFLAGS and #undef STATIC_PRECOMPUTATION if using basic config
Summary:
- Keep LDFLAGS if --coverage
- Undefine STATIC_PRECOMPUTATION if using the basic config
This is a backport of secp256k1's PR561
Test Plan:
make check
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D4996