Export: Test error on export of target and export with mixed paths.

If the export is installed to a DESTINATION path which is absolute,
the LIBRARY DESTINATION must also be absolute.

The LIBRARY DESTINATION is used as the IMPORTED_LOCATION of the generated
IMPORTED target, and it may conditionally contain a reference to
${_IMPORT_PREFIX}, which is only generated if exported to a relative
path.
cmPolicies: Remove superfluous newline.
Export: Diagnose issues in INCLUDES DESTINATION while generating (#15258).

The _IMPORT_PREFIX variable is generated in export files only if the
installation destination is relative.  If packagers set the path to
an absolute destination, the generated file must not have any
references to the _IMPORT_PREFIX as it is undefined.

A use of _IMPORT_PREFIX is generated for content passed to
install(TARGETS) in the INCLUDES DESTINATION component, if the
content is a relative path.  Issue a message to notify the user that
that the generated file is unusable, and introduce policy CMP0057 to
control the severity of the message.
Export: Issue error if generating unusable export file (#15258).

Diagnose presence of $<INSTALL_PREFIX> in exported target properties,
as can appear in user code, or generated for relative paths inside
a $<INSTALL_INTERFACE> generator expression.

The case of use of install(TARGETS) with a relative path in
INCLUDES DESTINATION would also be caught by this diagnostic, but
that is handled separately because in that case the $<INSTALL_PREFIX>
is generated by cmake internally, so a more-specific diagnostic is
desired.
Merge topic 'diagnose-invalid-_IMPORT_PREFIX-generation' into next

76a53049 Export: Issue error if generating unusable export file (#15258).
aab516bd Export: Diagnose issues in INCLUDES DESTINATION while generating (#15258).
4bf555b0 cmPolicies: Remove superfluous newline.
67f9fc26 Export: Test error on export of target and export with mixed paths.