Makefile: inprovements for bundle

1. Decide whether to use `bundle` or `jekyll` based on the presence of
   `Gemfile` or `Gemfile.lock`

2. Use `$(SHELL)` to call `bin/knit_lessons.sh`: this let's us avoid relying
   on `/usr/bin/env` to find `bash`.

3. New `bundle` target for building `.vendor/bundle` directory with
   required gems:

   a. This is an alias for `.vendor/bundle` target which depends on
   Gemfile and Gemfile.lock
   b. It installs/updates bundled gems only if necessary

4. Empty Gemfile and Gemfile.lock targets for lessons that don't use
   them.

5. `clean` target: remove `.vendor` directory

Makefile

@@ -3,10 +3,15 @@
 
 # Settings
 MAKEFILES=Makefile $(wildcard *.mk)
-JEKYLL=bundle config --local set path .vendor/bundle && bundle install && bundle update && bundle exec jekyll
 PARSER=bin/markdown_ast.rb
 DST=_site
 
+ifneq (, $(wildcard Gemfile*))
+  JEKYLL := bundle exec jekyll
+else
+  JEKYLL := jekyll
+endif
+
 # Check Python 3 is installed and determine if it's called via python3 or python
 # (https://stackoverflow.com/a/4933395)
 PYTHON3_EXE := $(shell which python3 2>/dev/null)
@@ -32,7 +37,7 @@ endif
 
 
 # Controls
-.PHONY : commands clean files install-rmd-deps
+.PHONY : clean
 
 # Default target
 .DEFAULT_GOAL := commands
@@ -68,6 +73,7 @@ clean :
 	@rm -rf ${DST}
 	@rm -rf .sass-cache
 	@rm -rf bin/__pycache__
+	@rm -rf .vendor
 	@find . -name .DS_Store -exec rm {} \;
 	@find . -name '*~' -exec rm {} \;
 	@find . -name '*.pyc' -exec rm {} \;
@@ -93,7 +99,7 @@ workshop-check :
 ## III. Commands specific to lesson websites
 ## =================================================
 
-.PHONY : lesson-check lesson-md lesson-files lesson-fixme
+.PHONY : lesson-check lesson-md lesson-files lesson-fixme install-rmd-deps
 
 # RMarkdown files
 RMD_SRC = $(wildcard _episodes_rmd/??-*.Rmd)
@@ -128,7 +134,7 @@ lesson-md : ${RMD_DST}
 
 _episodes/%.md: _episodes_rmd/%.Rmd install-rmd-dependencies
 	@mkdir -p _episodes
-	@bin/knit_lessons.sh $< $@
+	@$(SHELL) bin/knit_lessons.sh $< $@
 
 ## * lesson-check     : validate lesson Markdown
 lesson-check : lesson-fixme
@@ -157,6 +163,27 @@ lesson-fixme :
 ## IV. Auxililary (plumbing) commands
 ## =================================================
 
-## * commands         : show all commands.
+.PHONY : commands bundle
+
+## * commands         : show all commands
 commands :
 	@sed -n -e '/^##/s|^##[[:space:]]*||p' $(MAKEFILE_LIST)
+
+## * bundle           : install Ruby gems specified in Gemfile / Gemfile.lock
+bundle : .vendor/bundle
+
+.vendor/bundle: Gemfile Gemfile.lock
+ifneq (, $(wildcard Gemfile*))
+	@bundle config --local set path '.vendor/bundle'
+	@bundle install
+	@bundle update
+	@touch .vendor/bundle
+else
+	@echo Can not create bundle: neither Gemfile nor Gemfile.lock found.
+endif
+
+# Gemfile and Gemfile.lock targets below do nothing.  This is intentional.
+Gemfile:
+
+Gemfile.lock:
+