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.

Start with the README Maven dependency example and compare its classifier usage with the Windows dependencies shown in the issue. Trace where ${javacpp.platform.extension} is expected to be defined, then update the documentation so the defining dependency or required usage is explicit and the example is unambiguous.