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
Generate the _IMPORT_PREFIX in the non-config export file.
Add the INSTALL_PREFIX genex.
documentation: handling of relative paths by include- and link_directories()

Alex
FindGTK2: Fix GTK2_LIBRARIES order for static gtk libraries

Before this, when creating GTK2_LIBRARIES, FindGTK2 added the GTK
dependencies in wrong order into GTK2_LIBRARIES.  With dynamic libraries
this is not a major problem, but when linking to static gtk libraries,
the linker outputs a lot of undefined symbols.  Reorder the calls that
append libraries to GTK2_LIBRARIES to respect dependency order.
Fix TARGET_PROPERTY target extractions.

We need to make sure we can export targets which have content such
as $<0:$<TARGET_PROPERTY:not_a_target,INTERFACE_INCLUDE_DIRECTORIES>

That means making not finding a target non-fatal here.
Move GetCompileDefinitions to cmTarget.
Process COMPILE_DEFINITIONS as generator expressions in QtAutomoc.

Fixes #13493.
Merge topic 'PackageConfigHelper_UsrMove'

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 'fix-automoc-compile-defs'

429e369 Process COMPILE_DEFINITIONS as generator expressions in QtAutomoc.
0e10782 Move GetCompileDefinitions to cmTarget.
Merge topic 'fix-relocatable-include-dirs'

34d1ade Add the INSTALL_PREFIX genex.
3a17197 Generate the _IMPORT_PREFIX in the non-config export file.
Merge topic 'DocumentRelativePathHandling'

a7d6ebb documentation: handling of relative paths by include- and link_directories()
Merge topic 'FindGTK2-lib-order'

399c3b8 FindGTK2: Fix GTK2_LIBRARIES order for static gtk libraries
Merge topic 'fix-TARGET_PROPERTY-extraction'

5daaa5c Fix TARGET_PROPERTY target extractions.