SourceLayout: build auth sub-libraries in abstraction

SourceLayout: build auth sub-libraries in abstraction

This changes the building process for auth sub-libraries:
 libbasic.la, libdigest.la, libnegotiate.la, libntlm.la
making them build from their own Makefiles.

That allows each sub-dir to be automatically included (or not) to the main
auth/libauth.la library.

TODO: (no necessarily in this order)
 * split out the classes into their own compile units (files)
 * add namespace Auth and per-protocol child areas
 * de-duplicate the repetitive code back into the parent classes