export(): Check targets exist at configure-time (#14608)

Commit 66b290e7 (export(): Process the export() command at generate
time., 2012-10-06 ) refactored export() so that it could evaluate
strings at generate-time. This was intended for evaluating target
properties, but that commit also removed a check for target
existence at configure-time. Restore that check and add a test for
this case.
Merge topic 'fix-export-segfault'

1cd1430 export(): Check targets exist at configure-time (#14608)