Set variables when empty, for real this time

Closes: aura#164

Makefile

@@ -4,8 +4,13 @@ POETRY_RUN_MANAGE = $(POETRY_RUN) ./manage.py
 
 .DEFAULT_GOAL := run.prod
 
-TANK_CALLBACK_BASE_URL ?= "${AURA_PROTO}://${AURA_HOST}/tank"
-DASHBOARD_CALLBACK_BASE_URL ?= "${AURA_PROTO}://${AURA_HOST}"
+ifndef TANK_CALLBACK_BASE_URL
+override TANK_CALLBACK_BASE_URL = "${AURA_PROTO}://${AURA_HOST}/tank"
+endif
+
+ifndef DASHBOARD_CALLBACK_BASE_URL
+override DASHBOARD_CALLBACK_BASE_URL = "${AURA_PROTO}://${AURA_HOST}"
+endif
 
 initialize: migrate loadfixtures
 	$(POETRY_RUN_MANAGE) collectstatic --clear --no-input