Make C++ standard user-selectable

[calcmogul]

C++ dashboard applications can use newer compilers and standard versions than C++17. native-utils appends the C++17 flag after the user's flags, so it always overrides the standard version to C++17.

I tried changing the standard version by adding it.cppCompiler.args.add('-std=c++20') and it.cppCompiler.args.remove('-std=c++17') to build.gradle. The former added the C++20 flag, but the latter didn't stop the C++17 flag from being added, since I assume that happens later.