Added the `C5T_DLIB` symbol.

Hi @mzhurovich, CC @yarmaksergey,

This #define-d symbol for dlib builds makes it possible to fix OS X builds.

Context:

There were some functions that were not supposed to be called from certain classes when those classes are used from dlib-based builds.
For example, with C5T_LOGGER, it can only be initialized from the "main" process, not from the dlib itself, while the dlib should use the passed-in and injected instance of C5T_LOGGER.
This worked on Linux so far "just" because that code path was never executed. (And executing it would have been a failure!)
OS X is stricter, and this was resulting in linkage errors.
With this fix, a clean solution is now possible, implemented, and tested.

I'll merge this one-liner PR in, and link to the respective fix in the other repo in the comment.

Thx, Dima

C5T / Current