NDK should have a #define for the NDK version

there are places where it would be helpful to know "am i building with the NDK?" but even more so, places where you want to know which version of the NDK. if we want to get support for the next/beta release into a project but they (reasonably enough) want to keep working with the current stable release, for example, it would help if they could say something like #if (__NDK__+0)>=15.