Make the API for adding targets string based so that it can easily
use cmGeneratorTarget.
Teach the cmIncludeCommand to generate the exported file at
configure-time instead if it is to be include()d.
The RunCMake.ExportWithoutLanguage test now needs a dummy header.h
file as expected error from export() is now reported after the
missing file error.
This property is generated only for targets which have recorded
policy CMP0022 as NEW, and a compatibility mode is added to
additionally export the old interfaces in that case too.
If the old interfaces are not exported, the generated export files
require CMake 2.8.12. Because the unit tests use a version which
is not yet called 2.8.12, temporarily require a lower version.