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>
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Status Job ID Name Coverage
  Build
failed #2336
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  Test
skipped #2338
allowed to fail
code_quality
skipped #2342
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container_scanning
skipped #2341
allowed to fail
dependency_scanning
skipped #2339
allowed to fail
license_management
skipped #2340
allowed to fail
sast
skipped #2337
test
 
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