Refactoring merged by commit v3.5.0-rc1~299 (Merge topic
'use-generator-target', 2015-10-20) in and around
commit v3.5.0-rc1~299^2~13 (cmExportSet: Store a cmGeneratorTarget,
2015-10-17) changed export sets to delay looking up actual targets and
stores only their names.  However, in InstallCommand::HandleExportMode
we need to lookup targets immediately to check them for
EXPORT_LINK_INTERFACE_LIBRARIES.  The check was accidentally made local
to the current directory, so if an export set contains a target from
another directory the lookup fails and CMake crashes.  Fix the check to
look up the target name globally, and tolerate when no target is found
just in case.

Reported-by: Kelly Thompson <kgt@lanl.gov>