Export list of tool/compiler variables to re-clear in prelude.

[emrainey]

Each component must export the list of variables it requires to be cleared at the end of the prelude before another module is parsed.

CONCERTO_VARIABLES += MY_VAR YOUR_VAR

In prelude, the variables will be $(sort $(CONCERTO_VARIABLES)) to remove duplicates and then unset or set to empty (depending on make version)