Remove unused Doxygen file
Don't add CFLAGS and LDFLAGS to pkg-config files

- See: https://lintian.debian.org/tags/pkg-config-bad-directive.html
Use consistent flavor tags
Add missing dependencies
Use consistent PREDEFINED in all Doxyfiles

- Exclude test directories from doxygen generation (common and xio)
Use mixed case man page install for all packages
Doxygen markup fixes

- Fixes lintian warnings on Debian (though a very recent doxygen version is
  needed)