Greatly improve Makefile

- Only recompile the tests that have changed (Using macros and
  auto-generated targets! Eat that, automake!)
- Compute file size and gziped size and display them
- Use the `.PHONY` directive where relevant
- Hide some minor commands and write out others clearer

Makefile

@@ -1,20 +1,29 @@
+.PHONY: all clean check-size
 SASSC=sass
+SASSC_ARGS=--style compressed
 PANDOC=pandoc
+PANDOC_ARGS=--css=../styling.css -V lang=en -V highlighting-css= --mathjax \
+            --smart --to=html5
 TEST_MD=$(wildcard test_md/*.md)
+TEST_HTML=$(patsubst test_md/%.md,test_html/%.html,$(TEST_MD))
 
-all: clean compile test
+all: styling.css $(TEST_HTML) check-size
 
-compile:
-	$(SASSC) -t compressed styling.sass styling.css
+styling.css: styling.sass
+	$(SASSC) $(SASSC_ARGS) styling.sass styling.css
 
-test: compile
-	mkdir -p test_html
-	$(foreach i,$(TEST_MD),\
-		$(PANDOC) $(i) -o $(patsubst test_md/%.md,test_html/%.html,$(i)) \
-		          --standalone -t html5 --mathjax --smart -V highlighting-css= \
-	              -V lang=en -c ../styling.css ;\
-	)
+check-size: styling.css
+	@echo "raw size:    $(shell stat -c%s styling.css) bytes"
+	@echo "gziped size:" \
+	      "$(shell gzip -c styling.css | wc -c | awk '{print $1;}') bytes"
+
+define TEST_TARGET
+$(patsubst test_md/%.md,test_html/%.html,$(1)): $(1)
+	@mkdir -p test_html
+	$(PANDOC) $(PANDOC_ARGS) $$< -o $$@
+endef
+
+$(foreach i,$(TEST_MD),$(eval $(call TEST_TARGET,$(i))))
 
 clean:
-	rm -rf test_html
-	rm -f styling.css
+	rm -rf styling.css test_html