maven, how to set system properties?

[Jon Haugsand]

I'm new to maven, and I wonder how one can set java system properties
in a maven project? In particular I have a local temporary directory
in my project tree:

project/
+ src
+ data
+ target
+ tmp
and so on

So when I need a new temporary file, using

File.createTempFile("xxx","data");

I get a file in java.io.tmpdir which is /tmp. I should like to set it
to the tmp directory listed above. The following magic in
build.properties does not work:

java.io.tmpdir=${basedir}/tmp