Added -Wmissing-braces to CXXFLAGS where supported by the compiler

Summary: as title

Test Plan: ran autoreconf -i, then checked ../configure and ../cmake on fresh build dirs

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2192