message: build: avoid spurious rebuilds of external deps.
  Fixes
diff: @@ -56,8 +56,9 @@ build-recipe = cd "$(THIRDPARTY_SRC)" && \
  FORCE:
  # Capture $(ENV_STR) deps to retrigger build when they change.
  +.PRECIOUS: $(THIRDPARTY_DEPS)/%.dep.env
  $(THIRDPARTY_DEPS)/%.dep.env: FORCE
  - @[ "$(cat $@)" != "$(ENV_STR)" ] && echo "$(ENV_STR)" > $@
  + @[ "$$(cat $@)" != "$(ENV_STR)" ] && echo "$(ENV_STR)" > $@ || echo "No need to rebuild $@"
  $(THIRDPARTY_DEPS)/%.dep: $(RECIPES)/%.sh $(THIRDPARTY_DEPS)/%.dep.env
  @+$(call build-recipe,)
  