Help: Update discussion of relocable packages in cmake-packages(7)

Explain at the beginning of the section the requirements for a package
to be relocatable to justify the rest of the section content.
Generalize example to use fictional package names instead of real ones,
especially because FindBoost provides no alternative yet.  Reword the
discussion to represent the preferred approach as "ideal" but also
suggest workarounds when find modules do not provide the imported
targets.

Help/manual/cmake-packages.7.rst

@@ -475,6 +475,10 @@ without installation.  Consumers of the build tree can simply ensure that the
 Creating Relocatable Packages
 -----------------------------
 
+A relocatable package must not reference absolute paths of files on
+the machine where the package is built that will not exist on the
+machines where the package may be installed.
+
 Packages created by :command:`install(EXPORT)` are designed to be relocatable,
 using paths relative to the location of the package itself.  When defining
 the interface of a target for ``EXPORT``, keep in mind that the include
@@ -509,34 +513,56 @@ This also applies to paths referencing external dependencies.
 It is not advisable to populate any properties which may contain
 paths, such as :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` and
 :prop_tgt:`INTERFACE_LINK_LIBRARIES`, with paths relevant to dependencies.
-That would hard-code into installed packages the include directory or library
-paths for dependencies **as found on the machine the package was made on**.
-
-That is, code like this is incorrect for targets which will be used to
-generate config file packages:
+For example, this code may not work well for a relocatable package:
 
 .. code-block:: cmake
 
   target_link_libraries(ClimbingStats INTERFACE
-    ${Boost_LIBRARIES} ${OtherDep_LIBRARIES}
+    ${Foo_LIBRARIES} ${Bar_LIBRARIES}
     )
   target_include_directories(ClimbingStats INTERFACE
-    "$<INSTALL_INTERFACE:${Boost_INCLUDE_DIRS};${OtherDep_INCLUDE_DIRS}>"
+    "$<INSTALL_INTERFACE:${Foo_INCLUDE_DIRS};${Bar_INCLUDE_DIRS}>"
     )
 
-Dependencies must provide their own :ref:`IMPORTED targets <Imported Targets>`
-which have their own :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` and
-:prop_tgt:`IMPORTED_LOCATION` populated appropriately.  Those
-:ref:`IMPORTED targets <Imported Targets>` may then be
-used with the :command:`target_link_libraries` command for ``ClimbingStats``.
+The referenced variables may contain the absolute paths to libraries
+and include directories **as found on the machine the package was made on**.
+This would create a package with hard-coded paths to dependencies and not
+suitable for relocation.
 
-That way, when a consumer uses the installed package, the
-consumer will run the appropriate :command:`find_package` command (via the
-find_dependency macro described below) to find
-the dependencies on their own machine and populate the
-:ref:`IMPORTED targets <Imported Targets>` with appropriate paths. Note that
-many modules currently shipped with CMake do not currently provide
-:ref:`IMPORTED targets <Imported Targets>`.
+Ideally such dependencies should be used through their own
+:ref:`IMPORTED targets <Imported Targets>` that have their own
+:prop_tgt:`IMPORTED_LOCATION` and usage requirement properties
+such as :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` populated
+appropriately.  Those imported targets may then be used with
+the :command:`target_link_libraries` command for ``ClimbingStats``:
+
+.. code-block:: cmake
+
+  target_link_libraries(ClimbingStats INTERFACE Foo::Foo Bar::Bar)
+
+With this approach the package references its external dependencies
+only through the names of :ref:`IMPORTED targets <Imported Targets>`.
+When a consumer uses the installed package, the consumer will run the
+appropriate :command:`find_package` commands (via the ``find_dependency``
+macro described above) to find the dependencies and populate the
+imported targets with appropriate paths on their own machine.
+
+Unfortunately many :manual:`modules <cmake-modules(7)>` shipped with
+CMake do not yet provide :ref:`IMPORTED targets <Imported Targets>`
+because their development pre-dated this approach.  This may improve
+incrementally over time.  Workarounds to create relocatable packages
+using such modules include:
+
+* When building the package, specify each ``Foo_LIBRARY`` cache
+  entry as just a library name, e.g. ``-DFoo_LIBRARY=foo``.  This
+  tells the corresponding find module to populate the ``Foo_LIBRARIES``
+  with just ``foo`` to ask the linker to search for the library
+  instead of hard-coding a path.
+
+* Or, after installing the package content but before creating the
+  package installation binary for redistribution, manually replace
+  the absolute paths with placeholders for substitution by the
+  installation tool when the package is installed.
 
 .. _`Package Registry`: