Use libunwind as an imported target

[UVV-gh]

When UNWIND_LIBRARY is used directly as a public dependency then
absolute path is stored in cmake config file. This is an issue when
glog is used as part of an SDK, which was built somewhere else. When
SDK is installed on developer's machine, cmake config contains a full
path to non-existent location. The solution is to find libunwind
during configure stage and store target name as a dependency, not a full path

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[sergiud]

Please sign the CLA first.

Second, please avoid code duplication. The code should be put into a find module which is installed along glog under <prefix>/share/glog/cmake. The package config would then use find_dependency to correctly locate libunwind.

[googlebot]

CLAs look good, thanks!

[UVV-gh]

@sergiud Thanks for the review. Could you please take a look if it looks better now?

[UVV-gh]

I figured there's still room for improvement here. For example, libunwind is not found in cross-compilation environment. Please hold off for a while

[UVV-gh]

Updated. Please take a look again

[UVV-gh]

@sergiud I was wondering if you have any other comments/suggestions on this..

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[sergiud]

@googlebot I consent.

[googlebot]

CLAs look good, thanks!

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