In the <dependencies/> section of the POM, you can declare the shared jar as a "system" scoped dependency. This scope recognizes a <systemPath/> child element, allowing you to point to the filesystem location on the local (build) filesystem.
See Maven "System" dependencies
example:
<project>
…
<dependencies>
<dependency>
<groupId>com.example</groupId>
<artifactId>someDependency</artifactId>
<version>1.0.1</version>
<scope>system</scope>
<systemPath>${tomcatHome}/shared/lib/someDependency-1.0.1.jar</systemPath>
</dependency>
</dependencies>
…
</project>