Update README.md

README.md

@@ -162,12 +162,9 @@ On OSX, you can use brew:
 % brew install protobuf cmake
 ```
 
-If want to use self-prepared protobuf library, setup below environment variables before build:
+If want to use self-prepared protobuf library, specify protbof prefix before build:
 ```
-% export PROTOBUF=<path_to_protobuf>
-% export PROTOC="$PROTOBUF/bin/protoc"
-% export PROTOBUF_LIBS="-L$PROTOBUF/lib -lprotobuf -D_THREAD_SAFE"
-% export PROTOBUF_CFLAGS="-I$PROTOBUF/include -D_THREAD_SAFE" 
+% cmake .. -DCMAKE_PREFIX_PATH=<prefix_path_to_protobuf>
 ```
 
 ### Build and Install SentencePiece