cmake: Require C++14

[chfast]

No description provided.

[codecov-commenter]

Codecov Report

Merging #189 (e8894a5) into master (d39e474) will decrease coverage by 0.05%.
The diff coverage is 66.66%.

@@             Coverage Diff             @@
##            master     #189      +/-   ##
===========================================
- Coverage   100.00%   99.94%   -0.06%     
===========================================
  Files           22       22              
  Lines         1699     1700       +1     
===========================================
  Hits          1699     1699              
- Misses           0        1       +1     

Flag	Coverage Δ
be	89.77% <66.66%> (-0.06%)	⬇️
default	99.76% <66.66%> (-0.06%)	⬇️
x86_64	89.31% <66.66%> (-0.06%)	⬇️

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Impacted Files	Coverage Δ
test/unittests/test_progpow.cpp	98.90% <66.66%> (-1.10%)	⬇️