cmake: fix cross-compilation with gRPC_BUILD_GRPC_CPP_PLUGIN=OFF (#30378)

commit 99752b173c reintroduced the cross-compilation issue which was fixed by da10b795e1

CMakeLists.txt

@@ -514,7 +514,7 @@ function(protobuf_generate_grpc_cpp_with_import_path_correction FILE_LOCATION IM
          --plugin=protoc-gen-grpc=${_gRPC_CPP_PLUGIN}
          ${_protobuf_include_path}
          ${REL_FIL}
-    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/${FILE_LOCATION} ${ABS_FIL} ${_gRPC_PROTOBUF_PROTOC} grpc_cpp_plugin
+    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/${FILE_LOCATION} ${ABS_FIL} ${_gRPC_PROTOBUF_PROTOC} ${_gRPC_CPP_PLUGIN}
     WORKING_DIRECTORY ${_gRPC_PROTO_SRCS_DIR}
     COMMENT "Running gRPC C++ protocol buffer compiler for ${IMPORT_PATH}"
     VERBATIM)

templates/CMakeLists.txt.template

@@ -498,7 +498,7 @@
            --plugin=protoc-gen-grpc=<%text>${_gRPC_CPP_PLUGIN}</%text>
            <%text>${_protobuf_include_path}</%text>
            <%text>${REL_FIL}</%text>
-      DEPENDS <%text>${CMAKE_CURRENT_SOURCE_DIR}/${FILE_LOCATION}</%text> <%text>${ABS_FIL}</%text> <%text>${_gRPC_PROTOBUF_PROTOC}</%text> grpc_cpp_plugin
+      DEPENDS <%text>${CMAKE_CURRENT_SOURCE_DIR}/${FILE_LOCATION}</%text> <%text>${ABS_FIL}</%text> <%text>${_gRPC_PROTOBUF_PROTOC}</%text> <%text>${_gRPC_CPP_PLUGIN}</%text>
       WORKING_DIRECTORY <%text>${_gRPC_PROTO_SRCS_DIR}</%text>
       COMMENT "Running gRPC C++ protocol buffer compiler for <%text>${IMPORT_PATH}</%text>"
       VERBATIM)