Fix clang warnings
There is one remaining warning in leveldb.
The unused -pthread warning is removed by D2219.
This diff fix all the other warnings.
Build with clang and check there is no warning (exceptions are
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2220