configure_package_config_file: force absolute paths for usr-move

The configure_package_config()_file() macro will now use
absolute paths for the PATH_VARS if the Config.cmake file
will be installed into /lib(64) or /usr/lib(64), since due to
the usr-move filesystem changes Config.cmake files installed
there may be found via two paths (once per symlink via
/lib(64) and once via /usr/lib ), and in this case
relative paths break.

Alex
configure_package_config_file(): fix indentation

This is a separate commit, so that the previous commit is smaller.

Alex
configure_package_config_file(): extend documentation

Alex
documentation: handling of relative paths by include- and link_directories()

Alex
Merge topic 'DocumentRelativePathHandling' into next

a7d6ebb documentation: handling of relative paths by include- and link_directories()
Merge topic 'PackageConfigHelper_UsrMove' into next

4cad848 configure_package_config_file(): extend documentation
37c4bc1 configure_package_config_file(): fix indentation
d477414 configure_package_config_file: force absolute paths for usr-move