How to include a variable in !ENTITY tag?

[Keith Kee]

Hi:
  I am new to ant, and is thinking of migrating from GNUMake environment. It
looks like it is possible to migrating from the current GNUmake environment
except one major problem: I cannot specify an environment variable to the
ENTITY tag like this.
  <!DOCTYPE project [
      <!ENTITY include SYSTEM "${CONFIG_DIR}/common.xml">
  ]>

  Can somebody point out if this is possible, or if there is a viable
workaround? I search the mailing archive and FAQ with no avail.

TIA,
keith