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
CMake Nightly Date Stamp
Check lazily for whether parameters are targets

The target_link_libraries command can evaluate lazily whether
a parameter is a target, so these commands should too.

In the case of target_include_directories, content is either an absolute
path or a target. So, if it is not an absolute path we can be sure
it is a target at generate-time (or report an error). That is done
by using the TARGET_PROPERTY command without first checking if the
target is defined.

There is the disadvantage that the case of

 target_compile_definitions(foo PRIVATE SOME_DEFINE)

now expands in the COMPILE_DEFINITIONS property to

 "$<$<TARGET_DEFINED:SOME_DEFINE>:$<TARGET_PROPERTY:SOME_DEFINE,INTERFACE_COMPILE_DEFINITIONS>;$<$<NOT:$<TARGET_DEFINED:SOME_DEFINE>>:SOME_DEFINE>"

This may be a reason to document that all-uppercase content
to target_compile_definitions is always treated as a compile
definition, never as a target (but not vice-versa). Content such
as 'SOME_DEFINE=bar' is already treated only as a define, as it
does not match the targetNameValidator.
Generate the _IMPORT_PREFIX in the non-config export file.
Add the INSTALL_PREFIX genex.
Merge topic 'fix-relocatable-include-dirs' into next

34d1ade Add the INSTALL_PREFIX genex.
3a17197 Generate the _IMPORT_PREFIX in the non-config export file.
f7db6d1 CMake Nightly Date Stamp
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
Merge topic 'interface-commands-lazy-target-check' into next

6fcdd6e Check lazily for whether parameters are targets