Add target existence checks to Unwind find module (#518)

* Add target existence checks to Unwind find module

The build systems of projects depending on Glog may call the Unwind find
module multiple times. In these cases, the current unwind find module tries
to create a duplicate unwind::unwind target, crashing the build. This
patch adds an existence check before target creation to fix this issue.
Signed-off-by: Michael Darr <mdarr@matician.com>

* Alphabetize contributor list

* Fix inconsistent CMake style
Signed-off-by: Michael Darr <medarr@email.wm.edu>