How to skip downloading a dependency when a property flag is set in maven

I'm having some trouble articulating this question so I will try my best to abstract away as much irrelevant detail as possible. If more detail is required please ask.

I have a project containing a pom which contains a dependency which is always downloaded and unziped into a directory when a user does a mvn clean install on that pom. However I would like to forego the downloading and unpacking of that dependency when a user passes in a property such as mvn clean install -Dcontent=false but does everything else in that pom.

For a lack of a better way of saying this, I'd like to know how to make a certain dependency optional in maven? Not optional in the sense described here: http://maven.apache.org/guides/introduction/introduction-to-optional-and-excludes-dependencies.html

but optional at build time as described above.

Edit:

Build Step

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-dependency-plugin</artifactId>
            <version>3.0.1</version>
            <executions>
                <execution>
                    <id>unpack</id>
                    <phase>compile</phase>
                    <goals>
                        <goal>unpack</goal>
                    </goals>
                    <configuration>
                        <artifactItems>
                            <artifactItem>
                                <groupId>com.company.random</groupId>
                                <artifactId>content</artifactId>
                                <version>${contentVersion}</version>
                                <type>zip</type>
                                <outputDirectory>contentdir/target</outputDirectory>
                            </artifactItem>
                        </artifactItems>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    <plugins>
</build>

@Mikita Currently this will always execute, how could I make this to execute only when -Dcontent=true