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Enable C++14

Description

Enable C++14

Spidermonkey 60 can only be compiled with C++14. This conflicts with building 0 A.D. with C++11, so upgrade to C++14 first.
This in itself should not drop compiler support on any platform.

Tested on Windows by: Stan
Tested on Linux by: Freagarach

Refs #5859
Refs #5862

Differential Revision: https://code.wildfiregames.com/D3124