ENH: add the IMPORT keyword to ADD_EXECUTABLE(), which generates an
"imported" executable target. This can then be used e.g. with
ADD_CUSTOM_COMMAND() to generate stuff. It adds a second container for
"imported" targets, and FindTarget() now takes an additional argument bool
useImportedTargets to specify whether you also want to search in the
imported targets or only in the "normal" targets.

Alex