Fix issues reported by Matteo Magrin.

* Wrap clog.h with extern "C" for C++ compatibility.
* Move CLOG_LEVEL_NAMES inside the #ifdef CLOG_MAIN (duplicate symbol error).

Also took the chance to improve the tests a little:

* Moved to test/ subdirectory to make it obvious they're not required files.
* Link C and C++ tests into a single executable to ensure that works.
* Add a simple hello world test to the C++ file.

This gives the same coverage as before (verifying clog.h works in C99 and
C++98 files) as well as making sure it is link-time compatible with a
cross-language project.