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.