Make c++14 standard the default for compilation

Summary:
As per title.
Note that this will make it difficult to run bitcoin-abc on OpenBSD with
the current documentation, due to the old gcc version which does not
support c++14.

Test Plan:

make check

Run gitian builds

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2462