how to declare a dependency that depends of target JRE ?

[nicolas de loof]

Hello,

I'm writing a shared utility project that uses backport-util-concurrent.
This dependency has 3 versions, for java 1.2/1.3, java 1.4 and Java 5.
How to set my POM dependencies so that projects that use my lib get the
expected backport in classpath ?
For now the only way I've found is to declare dependency on backport as
optional.

Nico.