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).
Merge topic 'install-export-usr-move' into next

0c727b9 install(EXPORT): Force absolute paths for usr-move