Maven complains that ${javacpp.platform.extension} is undefined

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): Windows Version 10.0.19042.870
• Mobile device (e.g. iPhone 8, Pixel 2, Samsung Galaxy) if the issue happens on mobile device:
• TensorFlow installed from (source or binary):
• TensorFlow version: 0.3.1
• Python version: N/A
• Installed using virtualenv? pip? conda?: N/A
• Bazel version (if compiling from source): N/A
• GCC/Compiler version (if compiling from source): N/A
• CUDA/cuDNN version: N/A
• GPU model and memory: N/A

Describe the problem

The readme file references the following Maven code:

<dependency>
  <groupId>org.tensorflow</groupId>
  <artifactId>tensorflow-core-api</artifactId>
  <version>0.3.0</version>
  <classifier>linux-x86_64${javacpp.platform.extension}</classifier>
</dependency>

but it's not clear who defines ${javacpp.platform.extension}

When I add the following dependencies to my code, the build fails complaining that ${javacpp.platform.extension} is undefined:

			<dependency>
				<groupId>org.tensorflow</groupId>
				<artifactId>tensorflow-core-platform</artifactId>
				<version>${tensorflow.version}</version>
			</dependency>
			<dependency>
				<groupId>org.tensorflow</groupId>
				<artifactId>tensorflow-core-api</artifactId>
				<classifier>windows-x86_64${javacpp.platform.extension}</classifier>
				<version>${tensorflow.version}</version>
			</dependency>
			<dependency>
				<groupId>org.tensorflow</groupId>
				<artifactId>tensorflow-core-platform-gpu</artifactId>
				<version>${tensorflow.version}</version>
			</dependency>

Can you please clarify who defines this variable and what dependency I need to add to get this definition? Thanks in advance.

README の Maven 依存関係の例から始め、classifier の使用方法を issue に示されている Windows 依存関係と比較します。${javacpp.platform.extension} がどこで定義される想定なのかを追跡し、その定義元の依存関係または必要な使用方法が明示され、例が曖昧にならないようにドキュメントを更新します。