doxygen changes:

doxygen changes:
- moved doxygen.cfg to doxygen.cfg.in and let configure to the substitution
  on unix. Adapted the namke files accordingly.
- Don't add doxygen as a dependency for libethereal and libui: As doxygen is
  an unconditional target, this would cause the applications to be rebuild
  every time make was called, even when nothing changed in the meantime.
  This means that by now we need to do "make doxygen" manually in case we
  want updated documentation.

svn path=/trunk/; revision=11238