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