On some OSX macport installation, GNU commands may be used in favor
of native commands and 'make install' fail with
'ln: invalid option -- 'h'"
As BSD ln support the GNU -n option as an equivalent of the -h option
it shoud be preferred over the specific BSD option.
This unifies the way the include directories are handled in the
CMakeLists.txt files.
Paths are now normally relative to the root of the source- or build dir.
They may still be relative for paths withing a plugin subdirectory but
should no longer be relative to paths outside of the current source- or
build-directory.
The previous approach had resultet in many different styles which are
hard to wrap ones head around if new to the build system.
It sometimes defined includes twice
By using relative paths, it was not possible to copy paste the paths
between different files.
