Well, what you can do is using Maven profiles to include conditional dependencies. Glassfish contains jsf API and implementation out of the box. So what you need for your project is to specify what API you want to code against, marking it as provided as you don't need to deploy them:
<profiles>
<profile>
<id>old-glassfish</id>
<dependencies>
<dependency>
<groupId>javax.faces</groupId>
<artifactId>jsf-api</artifactId>
<version>2.0</version>
<scope>provided</scope>
</dependency>
</dependencies>
</profile>
<profile>
<id>new-glassfish</id>
<dependencies>
<dependency>
<groupId>javax.faces</groupId>
<artifactId>jsf-api</artifactId>
<version>2.1</version>
<scope>provided</scope>
</dependency>
</dependencies>
</profile>
</profiles>
I don't do Glassfish, but also you should change the provided JSF libraries to stablish the one you're interested in. There are some threads here at SO which explain what you have to do to update them.
Otherways, you could also use your own JSF implementation for each application, configuring the server properly not to use its bundled libraries.
See also: