Call “npm install” automatically if package.json changed (#505)

This adds a stamp file which is used to detect whether the `package.json`
file got updated since the last `npm install`.  If so, `npm install` is run
again to update all modules to the version described in `package.json`.
This happens as a dependency of only those modules which actually need some
npm-installed module.

Setting the corresponding make variable to the empty string disables the
feature, which is used by the `make` invocation in the `prepublish` script
inside `package.json` to avoid infinite loops.  It can also be used by
developers working in an environment with reduced connectivity, as long as
they know what they are doing.

Makefile

@@ -13,19 +13,24 @@ export UGLIFYJS = $(realpath ./node_modules/.bin/uglifyjs) \
 	--beautify \
 	ascii_only=true,beautify=false
 
-setup:
+# The prepublish script in package.json will override the following variable,
+# setting it to the empty string and thereby avoiding an infinite recursion
+NIS = .npm-install.stamp
+
+$(NIS) setup: package.json
 	npm install
+	@touch $(NIS)
 
-lint: katex.js server.js cli.js $(wildcard src/*.js) $(wildcard test/*.js) $(wildcard contrib/*/*.js) $(wildcard dockers/*/*.js)
-	./node_modules/.bin/eslint $^
+lint: $(NIS) katex.js server.js cli.js $(wildcard src/*.js) $(wildcard test/*.js) $(wildcard contrib/*/*.js) $(wildcard dockers/*/*.js)
+	./node_modules/.bin/eslint $(filter-out *.stamp,$^)
 
-build/katex.js: katex.js $(wildcard src/*.js)
+build/katex.js: katex.js $(wildcard src/*.js) $(NIS)
 	$(BROWSERIFY) $< --standalone katex > $@
 
 build/katex.min.js: build/katex.js
 	$(UGLIFYJS) < $< > $@
 
-build/katex.less.css: static/katex.less $(wildcard static/*.less)
+build/katex.less.css: static/katex.less $(wildcard static/*.less) $(NIS)
 	./node_modules/.bin/lessc $< $@
 
 build/katex.min.css: build/katex.less.css
@@ -78,10 +83,10 @@ compress: build/katex.min.js build/katex.min.css
 	@printf "Minified, gzipped css: %6d\n" "${CSSSIZE}"
 	@printf "Total:                 %6d\n" "${TOTAL}"
 
-serve:
+serve: $(NIS)
 	node server.js
 
-test:
+test: $(NIS)
 	JASMINE_CONFIG_PATH=test/jasmine.json node_modules/.bin/jasmine
 
 PERL=perl
@@ -94,7 +99,7 @@ extended_metrics:
 	cd metrics && $(PERL) ./mapping.pl | $(PYTHON) ./extract_tfms.py | $(PYTHON) ./extract_ttfs.py | $(PYTHON) ./format_json.py --width > ../src/fontMetricsData.js
 
 clean:
-	rm -rf build/*
+	rm -rf build/* $(NIS)
 
-screenshots: test/screenshotter/unicode-fonts
+screenshots: test/screenshotter/unicode-fonts $(NIS)
 	dockers/Screenshotter/screenshotter.sh