Add cmake/CMakeLists.txt for compatibility (#9596)

CMakeLists.txt

@@ -23,6 +23,15 @@ endif()
 # Project
 project(protobuf C CXX)
 
+if(protobuf_DEPRECATED_CMAKE_SUBDIRECTORY_USAGE)
+  if(CMAKE_PROJECT_NAME STREQUAL "protobuf")
+    get_filename_component(CMAKE_SOURCE_DIR ${CMAKE_SOURCE_DIR} DIRECTORY)
+  endif()
+  get_filename_component(CMAKE_CURRENT_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR} DIRECTORY)
+  get_filename_component(PROJECT_SOURCE_DIR ${PROJECT_SOURCE_DIR} DIRECTORY)
+  get_filename_component(protobuf_SOURCE_DIR ${protobuf_SOURCE_DIR} DIRECTORY)
+endif()
+
 # Add c++11 flags
 if (CYGWIN)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")

cmake/CMakeLists.txt

@@ -0,0 +1,9 @@
+cmake_minimum_required(VERSION 3.5)
+
+message(WARNING "Calling of cmake with source directory set to \"cmake\" subdirectory of Protocol Buffers project is deprecated. Top-level directory of Protocol Buffers project should be used instead.")
+
+project(protobuf C CXX)
+
+set(protobuf_DEPRECATED_CMAKE_SUBDIRECTORY_USAGE TRUE)
+
+include(../CMakeLists.txt)