fixup! Export: Diagnose issues in INCLUDES DESTINATION while
fixup! Export: Issue error if generating unusable export file (#15258).
fixup! Export: Issue error if generating unusable export file (#15258).
Merge topic 'diagnose-invalid-_IMPORT_PREFIX-generation' into next

d8b6a60f fixup! Export: Issue error if generating unusable export file (#15258).
a8edbcae fixup! Export: Issue error if generating unusable export file (#15258).
c8645a32 fixup! Export: Diagnose issues in INCLUDES DESTINATION while
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.

Extend policy CMP0057 to cover this diagnostic.
Merge topic 'diagnose-invalid-_IMPORT_PREFIX-generation' into next

9c32070a Export: Issue error if generating unusable export file (#15258).
29f92485 Export: Diagnose issues in INCLUDES DESTINATION while generating (#15258).