improve makefile (use latexmk)

Makefile

+LATEXMKOPTS=#-pdf #uncomment for forcing pdflatex
+LATEXMK=latexmk $(LATEXMKOPTS)
 
-slides.pdf: $(wildcard *.tex) $(wildcard *.sty) $(wildcard media/*)
-	lualatex slides
-	lualatex slides
+slides.pdf: $(wildcard *.tex) $(wildcard theme/*.sty) $(wildcard media/*)
+	$(LATEXMK) slides
 
 clean:
-	echo "Cleaning latex is almost impossible to do right; use `git clean`."
+	$(LATEXMK) -C slides