Colin is correct that a profile should be used. However, his answer hard-codes the target directory in the profile. An alternate solution would be to add a profile like this:
<profile>
<id>alternateBuildDir</id>
<activation>
<property>
<name>alt.build.dir</name>
</property>
</activation>
<build>
<directory>${alt.build.dir}</directory>
</build>
</profile>
Doing so would have the effect of changing the build directory to whatever is given by the alt.build.dir property, which can be given in a POM, in the user's settings, or on the command line. If the property is not present, the compilation will happen in the normal target directory.
Source: Stackoverflow.com