<include:foo --optional>
There a couple of old school ways of achieving the same thing.
<if -e --file ($config.INCLUDE_DIR + '/foo.inc')>
<include:foo>
</if>
or
<try>
<include:foo>
<catch>
</try>
Either way you slice it, the old school way seems forced. The --optional tag is admittedly syntactic sugar, but a welcome sweetener!