Simplify external protobuf script

Author: FloopCZ

tensorflow_cc/cmake/ProtobufExternal.cmake

@@ -18,17 +18,14 @@ ExternalProject_Add(
   protobuf-external
   PREFIX protobuf
   URL "${PROTOBUF_ARCHIVE}"
-  BINARY_DIR "${CMAKE_CURRENT_BINARY_DIR}/protobuf"
   CMAKE_CACHE_ARGS
     "-DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}"
     "-Dprotobuf_BUILD_TESTS:BOOL=OFF"
     "-Dprotobuf_BUILD_EXAMPLES:BOOL=OFF"
     "-DCMAKE_CXX_COMPILER:STRING=${CMAKE_CXX_COMPILER}"
   SOURCE_SUBDIR cmake
-  BUILD_ALWAYS 1
-  STEP_TARGETS build
   INSTALL_COMMAND ""
 )
 
-install(SCRIPT "${CMAKE_CURRENT_BINARY_DIR}/protobuf/cmake_install.cmake")
+install(SCRIPT "${CMAKE_CURRENT_BINARY_DIR}/protobuf/src/protobuf-external-build/cmake_install.cmake")