This allows USES to add targets easily and have them run whenever they want without touching bsd.port.mk.
To add a target that runs around the time post-configure run, do:
_USES_configure+= 700:my-post-configure my-post-configure: do something
To fine tune when the target is ran, look at the values in the *_SEQ variables at the end of bsd.port.mk, and the other USES.