Use target gtest instead of GTEST_LIB and GTEST_INCLUDE_DIR

This simplifies usage of GoogleTest, since independent handling of
GTEST_LIB and GTEST_INCLUDE_DIR is not necessary anymore.

Additionally CMake creates a dependency now between target gtest and all
test applications using it. This improves build process when building
GoogleTest from source code. When any test application is built,
GoogleTest library is automatically rebuilt if necessary now for
instance.

common/test-core/CMakeLists.txt

@@ -82,6 +82,13 @@ if (GTEST_SRC_DIR)
     target_compile_options(gtest PRIVATE -Wno-missing-declarations)
   endif()
   target_include_directories(gtest PUBLIC ${GTEST_INCLUDE_DIR} ${GTEST_SRC_DIR})
+else()
+  add_library(gtest UNKNOWN IMPORTED GLOBAL)
+  set_target_properties(gtest PROPERTIES
+    IMPORTED_LOCATION ${GTEST_SHARED_LIB}
+    INTERFACE_INCLUDE_DIRECTORIES ${GTEST_INCLUDE_DIR}
+    INTERFACE_LINK_LIBRARIES ${GTEST_MAIN_LIB}
+  )
 endif()
 if (GMOCK_SRC_DIR)
   set (lib_gmock_SOURCES ${GMOCK_SRC})