Fix: Use C++17 as standard in CMake script

[spnda]

Also lets the script default to the Vulkan-Headers submodule for ease-of-use.

[SaschaWillems]

Sorry for the late reply, but what's the reason behind the option to use Vulkan headers others than the one from this submodule? The application should always be tied to a given header revision.

[spnda]

I don't really know, that was already a thing before but it never defaulted to the submodule properly. Now when you build it it will always use the submodule as expected. Probably something about the SDK build process sharing a single headers clone.