In ActiveMQ as well as some other projects like Qpid JMS we pull in the JMS spec classes from Apache Geronimo JARs, the 1.1 APIs are available in this dependency:
<dependency>
<groupId>org.apache.geronimo.specs</groupId>
<artifactId>geronimo-jms_1.1_spec</artifactId>
<version>1.1.1</version>
</dependency>
For JMS 2 APIs you'd need to use a different dependency, for instance
<dependency>
<groupId>org.apache.geronimo.specs</groupId>
<artifactId>geronimo-jms_2.0_spec</artifactId>
<version>1.0-alpha-2</version>
</dependency>
These are both Apache 2.0 licensed dependencies.
Another option which is not Apache licensed is here as others have pointed out.
<dependency>
<groupId>javax.jms</groupId>
<artifactId>javax.jms-api</artifactId>
<version>2.0.1</version>
</dependency>