Handle usr-move without forcing absolute paths (#14041)

In commit 0c727b90 (install(EXPORT): Force absolute paths for usr-move,
2013-03-08) and commit d4774140 (configure_package_config_file: force
absolute paths for usr-move, 2013-01-24) we supported Linux
distributions implementing the "/usr move" by assuming that installation
to (/usr)?/lib(64)? represents a non-relocatable system package.

When cross-compiling one may prepare a package for installation into a
system location on a target machine but install the package files on the
*host* machine inside another path for use with CMAKE_FIND_ROOT_PATH.
In this case the package development files must still be relocatable.

Handle "/usr move" with a new approach that works with relocatable
files.  Teach configure_package_config_file and install(EXPORT) to
generate special logic in a package configuration file or targets file
for installation under (/usr)?/lib(64)?.  Teach the file to recognize
when it is loaded through a symlink that refers to the same realpath as
its original install destination.  In such a case, use the original
install prefix.  Otherwise, compute the prefix relative to the current
file location to make it relocatable.
Merge topic 'usr-move-relocatable'

6c613b4 Handle usr-move without forcing absolute paths (#14041)