Make WINDOW_G configurable
- Make WINDOW_G configurable
This makes WINDOW_G a configurable value in the range of [2..24].
The upper limit of 24 is a defensive choice. The code is probably
correct for values up to 27 but those larger values yield in huge
tables (>= 256MiB), which are i) unlikely to be really beneficial
in practice and ii) increasingly difficult to test.
- Clean up ./configure help strings
This is a backport of secp256k1 PR596
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D5030