doxygen: fix list of source file directories
cmake: set include sir "marsyas/src" once for all subdirs
unit tests: update include directives to reflect new policy
unit_tests: fix building with static marsyas library

There appears to be no need for linking directly to the output path
of marsyas static library, at least on Linux. Moreover, that disables
the CMake functionality for transitive linking. So rather link to
the "marsyas" target itself.

Let's see if this builds on Windows and OS X as well...