Cycles: Remove glog dependency, redirect logs to CLOG

* Add own simple logging system to replace glog, which is no longer
  maintained by Google.
* When building in Blender, integrate with CLOG and print all messages
  through that system instead.
* --log cycles now replaces --debug-cycles. The latter still works but
  is no longer documented.

Pull Request: https://projects.blender.org/blender/blender/pulls/140244

extern/CMakeLists.txt

@@ -86,7 +86,7 @@ if(WITH_LIBMV)
   add_subdirectory(ceres)
 endif()
 
-if(WITH_LIBMV OR WITH_GTESTS OR (WITH_CYCLES AND WITH_CYCLES_LOGGING))
+if(WITH_LIBMV OR WITH_GTESTS)
   if(NOT WITH_SYSTEM_GFLAGS)
     add_subdirectory(gflags)
   endif()