Create IMPORTED CMake targets for executables.

Create IMPORTED CMake targets for executables.

Although IMPORTED executables are not extra special, this is more future-proof
in terms of both future CMake features and future our needs - it is possible
that we would want to add a property to an executable at TARGET scope, which
would not be possible if it is just a path.

Change-Id: I649c601e004b21603c5fa97de0b7c397813ed68d
Reviewed-by: Brad King <brad.king@kitware.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>