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.
First, it prevents a NULL dereference and second it reiterates that
targets without languages are not supported by CMake.
Add a RunCMake.ExportWithoutLanguage test exporting a library without a
languages.