A "native library" is a project which is to be built as a shared library on most targets and as a framework on Darwin. (Currently this is only the case for apple-apple-apple.) In other words, it is to be built as the most appropriate native equivalent of a traditional shared library (see library.make and framework.make).
NATIVE_LIBRARY_NAMEshould be the name of the native library, without the 'lib'. All the other variables are the same as the ones used in libraries and frameworks.
To compile something against a native library, you can use
ADDITIONAL_NATIVE_LIBS += MyLibrary
This will be converted into -lMyLibrary link flag on for most
targets and into -framework MyLibrary link flag for
apple-apple-apple.