[cmake] define STRERROR_R_CHAR_P

Summary:
This var is used in sock.cpp (syserror.cpp after D16209) to pick the correct flavor of strerror_r but is never defined.
Define it.

This solution is based on https://github.com/hebasto/bitcoin/pull/6

Test Plan:

cmake .. -GNinja
ninja

Check the var is set in build/CMakeCache.txt and build/src/config/bitcoin-config.h

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16212