Print useful error message when signature is ill-formed.
Merge topic 'install-interface-includes' into next

ae93414 Print useful error message when signature is ill-formed.
Add a convenient way to add the includes install dir to the INTERFACE.

Export the INCLUDES DESTINATION without appending to the
INTERFACE_INCLUDE_DIRECTORIES of the target itself. That way, a target
can be exported multiple times with different INCLUDES DESTINATION
without unintended cross-pollution of export sets.
Merge topic 'install-interface-includes' into next

650e61f Add a convenient way to add the includes install dir to the INTERFACE.