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.
export(): Handle multiple dependent export sets.

The export-sets topic, merged in commit 49c7b649 (Merge topic
'export-sets', 2012-10-01) changed install(EXPORT) to allow
exporting targets whose dependents are exported separately
to different locations. Doing the same for export() was not
possible because the export() command was executed at
configure-time.

Now that export() is also executed at generate-time, make it
possible to export to multiple dependent export sets.
Merge topic 'export-at-generate-time' into next

a96ad39 export(): Handle multiple dependent export sets.
8aaeeea export(): Process the export() command at generate time.