Enable benchmark building by default and fix benchmarks for gcc 6+

Summary: As per title. this is backport of upstream PR480 and PR490 by Pieter Wuille.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1250