export(): Process the export() command at generate time.

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.