Re-organize structure of GLog/GFlags CMake libraries

The idea is to split them into two separate targets and have dedicated include
directories list for each of them in order to avoid some annoying include header
modifications in comparison with upstream.

Reviewers: campbellbarton, juicyfruit

Differential Revision: https://developer.blender.org/D1706

build_files/cmake/Modules/GTestTesting.cmake

@@ -34,7 +34,8 @@ macro(BLENDER_SRC_GTEST_EX NAME SRC EXTRA_LIBS DO_ADD_TEST)
 		                      extern_gtest
 		                      # needed for glog
 		                      ${PTHREADS_LIBRARIES}
-		                      extern_glog)
+		                      extern_glog
+		                      extern_gflags)
 		set_target_properties(${NAME}_test PROPERTIES
 		                      RUNTIME_OUTPUT_DIRECTORY         "${TESTS_OUTPUT_DIR}"
 		                      RUNTIME_OUTPUT_DIRECTORY_RELEASE "${TESTS_OUTPUT_DIR}"