Cmake for the "mutate" branch needs C++ 14 support

[jamesguo71]

On macOS, llvm 13.

I was making my way through the tutorial and had a build error in the "mutate branch", beginning with:

/opt/homebrew/opt/llvm/include/llvm/Support/SwapByteOrder.h:152:13: error: no template named 'enable_if_t' in namespace 'std'; did you mean 'enable_if'?
inline std::enable_if_t<std::is_enum::value, T> getSwappedBytes(T C) {
~~~~~^~~~~~~~~~~
enable_if

Turns out this construct is only supported from C++ 14. So I added this to the Cmake file:

set(CMAKE_CXX_STANDARD 14)

and fixed the build error.

Hope this helps.