Only run `npm install` when needed

Makefile

# An example of how to only run `npm install` only when package.json or package-lock.json
# have changed. This might be helpful for situation where you run `npm install` every time
# and app or service lauches (e.g. for local development), but the install takes a long time.
#
# This approach creates a tracking file called `make_installed`. Whenever `npm install` is
# invoked, this file is touched. If the file is newer then the modification date on either
# package.json or package-lock.json then no installation is done. If it is older, 
# then installation is done.
# 
# This also works with git. When you checkout a previous commit the modification date for
# the checked out files will be the current date, not the date it was commited.
#
node_modules/make_installed: package.json package-lock.json
	npm install
	touch node_modules/make_installed

install: node_modules/make_installed