Use export in make recipe

example/Makefile

@@ -11,7 +11,6 @@ BUILDDIR      ?= _build
 # logging variables
 DEBUG ?= 0
 LOGLEVEL =? WARNING
-export LOGLEVEL
 
 # Internal variables.
 PAPEROPT_a4     = -D latex_paper_size=a4
@@ -51,6 +50,7 @@ clean:
 	-rm -rf $(BUILDDIR)/*
 
 html:
+	export LOGLEVEL=$(LOGLEVEL)
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."