[secp256k1] Allow to use external default callbacks

Summary:

• Replace CHECKs for no_precomp ctx by ARG_CHECKs without a return
• Allow usage of external default callbacks
• Include stdio.h and stdlib.h explicitly in secp256k1.c
• Add secp256k1_ prefix to default callback functions
• Add missing #(un)defines to base-config.h
• cmake build

This is a backport of libsecp256k1 PR595

Test Plan:
Ran unit tests, it doesn't break anythign at least.

cmake -GNinja .. -DSECP256K1_ENABLE_EXTERNAL_DEFAULT_CALLBACKS=ON
ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5308