install(EXPORT): Force absolute paths for usr-move
If the absolute install(EXPORT) destination for the CMAKE_INSTALL_PREFIX used during configuration is under (/usr)?/lib(64)? then assume the current build is for a system package installation instead of a relocatable distribution. Generate an absolute path for _IMPORT_PREFIX in the target exports file instead of generating code to compute the value relative to the file location. This is necessary for distributions implementing a move to /usr such as: https://wiki.archlinux.org/index.php/DeveloperWiki:usrlib "All files in the /lib directory have been moved to /usr/lib and now /lib is a symlink to usr/lib." The relative path computation is not reliable because the targets file could be installed through cross-prefix a symlink and loaded without it or vice versa. A similar change was made for package configuration file generation by commit d4774140 (configure_package_config_file: force absolute paths for usr-move, 2013-01-24).
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