When cmExportFileGenerator::SetImportLinkInterface exports the old
LINK_INTERFACE_LIBRARIES property values it skips doing so for
non-linkable targets because target->GetLinkInterface returns NULL for
such targets.
Since cmExportFileGenerator::PopulateInterfaceLinkLibrariesProperty
looks at the INTERFACE_LINK_LIBRARIES property directly instead of using
the computed link interface, teach it to skip exporting the property if
target->IsLinkable returns false.
Extend the RunCMake.CMP0022 test with a case covering this. Simply
export an executable target that links to a library that is not
exported.
Brad King