Document setting C++ compiler flags in GradleRIO

[calcmogul]

Here's some examples from my team's build.gradle:

nativeUtils.platformConfigs.named("linuxathena").configure {
    it.cppCompiler.args.add('-Wall')
    it.cppCompiler.args.add('-Wextra')
    it.cppCompiler.args.add('-pedantic')
    it.cppCompiler.args.add('-Werror')
    it.cppCompiler.args.add('-Wno-unused-parameter')
    // Suppress warning from incorrect [[deprecated]] attribute usage in
    // CANSparkMax.h
    it.cppCompiler.args.add('-Wno-attributes')
}

if (OperatingSystem.current().isWindows()) {
    nativeUtils.platformConfigs.named(wpi.platforms.desktop).configure {
        it.cppCompiler.args.add('/W3')
        it.cppCompiler.args.add('/WX')
    }
} else {
    nativeUtils.platformConfigs.named(wpi.platforms.desktop).configure {
        it.cppCompiler.args.add('-Wall')
        it.cppCompiler.args.add('-Wextra')
        it.cppCompiler.args.add('-pedantic')
        it.cppCompiler.args.add('-Werror')
        it.cppCompiler.args.add('-Wno-unused-parameter')
    }
}

nativeUtils.platformConfigs.named("linuxx86-64").configure {
    it.linker.args.add('-lstdc++fs')
}

if (project.hasProperty("asan")) {
    nativeUtils.platformConfigs.named("linuxx86-64").configure {
        it.cppCompiler.args.add('-g3')
        it.cppCompiler.args.add('-fsanitize=address')
        it.linker.args.add('-fsanitize=address')
    }
}

[Daltz333]

Not sure where to put this. Welcome to suggestions.

[sciencewhiz]

Fixed by #2393