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