CMake: Add option to link against system-wide Gflags library

It is disabled by default, so should not affect existing configurations.

Main benefits of this goes as:

- Linux distros can use that to avoid libraries duplication and link
  blender package against gflags package from the system.

- It it easier to test whether Blender works with updated version of
  Gflags prior to re-bundling the library.

extern/CMakeLists.txt

@@ -99,7 +99,9 @@ if(WITH_LIBMV)
 endif()
 
 if(WITH_LIBMV OR WITH_GTESTS OR (WITH_CYCLES AND WITH_CYCLES_LOGGING))
-	add_subdirectory(gflags)
+	if (NOT WITH_SYSTEM_GFLAGS)
+		add_subdirectory(gflags)
+	endif()
 	add_subdirectory(glog)
 endif()