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.
Cmake module looks for libunwind and libunwind-PLAT, where PLAT is one of
supported platforms.
Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>