Add LibUnwind as a dependency of glog::glog (#101)

Summary:
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Pull Request resolved: #101

X-link: facebookincubator/katran#172

X-link: facebook/mvfst#273

X-link: https://github.com/fairinternal/AIRStore/pull/38

`LibUnwind` is a dependency of `glog` according to [objdump](https://www.internalfb.com/intern/skycastle/run/117093590319624253/artifact/actionlog.117093590478433674.stdout). This diff adds ${LIBUNWIND_LIBRARY} to the CMake imported library `glog::glog` as an element in the `IMPORTED_LINK_INTERFACE_LIBRARIES` property.

Without this diff, there will be a linker error like this:
```
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libglog.so: undefined reference to symbol '_Ux86_64_getcontext'
//usr/lib/x86_64-linux-gnu/libunwind.so.8: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
```

Reviewed By: Wilfred

Differential Revision: D42509278

fbshipit-source-id: ea80449ccc6b91d658bbe9cee954fc8a4a17926f

build/fbcode_builder/CMake/FindGlog.cmake

@@ -38,6 +38,12 @@ if (NOT TARGET glog::glog)
   find_package(Gflags)
   if(GFLAGS_FOUND)
     message(STATUS "Found gflags as a dependency of glog::glog, include=${LIBGFLAGS_INCLUDE_DIR}, libs=${LIBGFLAGS_LIBRARY}")
-    set_target_properties(glog::glog PROPERTIES IMPORTED_LINK_INTERFACE_LIBRARIES ${LIBGFLAGS_LIBRARY})
+    set_property(TARGET glog::glog APPEND PROPERTY IMPORTED_LINK_INTERFACE_LIBRARIES ${LIBGFLAGS_LIBRARY})
+  endif()
+
+  find_package(LibUnwind)
+  if(LIBUNWIND_FOUND)
+    message(STATUS "Found LibUnwind as a dependency of glog::glog, include=${LIBUNWIND_INCLUDE_DIR}, libs=${LIBUNWIND_LIBRARY}")
+    set_property(TARGET glog::glog APPEND PROPERTY IMPORTED_LINK_INTERFACE_LIBRARIES ${LIBUNWIND_LIBRARY})
   endif()
 endif()