message: Update absl to 20200923.3
diff: @@ -26,7 +26,7 @@ endforeach()
  ExternalProject_add(
  absl
  GIT_REPOSITORY https://github.com/abseil/abseil-cpp.git
  - GIT_TAG 20200923.2
  + GIT_TAG 20200923.3
  PREFIX absl
  PATCH_COMMAND git apply ${PROJECT_SOURCE_DIR}/absl_c++11.patch
  CMAKE_ARGS "${CMAKE_ARGS}"
  