Only print make venv message when needed. (python#867)

* Only print `make venv` message when needed.

* Remove duplicated code.

Author: ezio-melotti

Makefile

@@ -52,44 +52,49 @@ venv:
 		echo "venv already exists."; \
 		echo "To recreate it, remove it first with \`make clean-venv'."; \
 	else \
+		$(MAKE) ensure-venv; \
+	fi
+
+ensure-venv:
+	@if [ ! -d $(VENVDIR) ] ; then \
 		$(PYTHON) -m venv $(VENVDIR); \
 		$(VENVDIR)/bin/python3 -m pip install --upgrade pip; \
 		$(VENVDIR)/bin/python3 -m pip install -r requirements.txt; \
 		echo "The venv has been created in the $(VENVDIR) directory"; \
 	fi
 
-html: venv
+html: ensure-venv
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 
-dirhtml: venv
+dirhtml: ensure-venv
 	$(SPHINXBUILD) -b dirhtml $(ALLSPHINXOPTS) $(BUILDDIR)/dirhtml
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/dirhtml."
 
-singlehtml: venv
+singlehtml: ensure-venv
 	$(SPHINXBUILD) -b singlehtml $(ALLSPHINXOPTS) $(BUILDDIR)/singlehtml
 	@echo
 	@echo "Build finished. The HTML page is in $(BUILDDIR)/singlehtml."
 
-pickle: venv
+pickle: ensure-venv
 	$(SPHINXBUILD) -b pickle $(ALLSPHINXOPTS) $(BUILDDIR)/pickle
 	@echo
 	@echo "Build finished; now you can process the pickle files."
 
-json: venv
+json: ensure-venv
 	$(SPHINXBUILD) -b json $(ALLSPHINXOPTS) $(BUILDDIR)/json
 	@echo
 	@echo "Build finished; now you can process the JSON files."
 
-htmlhelp: venv
+htmlhelp: ensure-venv
 	$(SPHINXBUILD) -b htmlhelp $(ALLSPHINXOPTS) $(BUILDDIR)/htmlhelp
 	@echo
 	@echo "Build finished; now you can run HTML Help Workshop with the" \
 	      ".hhp project file in $(BUILDDIR)/htmlhelp."
 
-qthelp: venv
+qthelp: ensure-venv
 	$(SPHINXBUILD) -b qthelp $(ALLSPHINXOPTS) $(BUILDDIR)/qthelp
 	@echo
 	@echo "Build finished; now you can run "qcollectiongenerator" with the" \
@@ -98,7 +103,7 @@ qthelp: venv
 	@echo "To view the help file:"
 	@echo "# assistant -collectionFile $(BUILDDIR)/qthelp/PythonDevelopersGuide.qhc"
 
-devhelp: venv
+devhelp: ensure-venv
 	$(SPHINXBUILD) -b devhelp $(ALLSPHINXOPTS) $(BUILDDIR)/devhelp
 	@echo
 	@echo "Build finished."
@@ -107,54 +112,54 @@ devhelp: venv
 	@echo "# ln -s $(BUILDDIR)/devhelp $$HOME/.local/share/devhelp/PythonDevelopersGuide"
 	@echo "# devhelp"
 
-epub: venv
+epub: ensure-venv
 	$(SPHINXBUILD) -b epub $(ALLSPHINXOPTS) $(BUILDDIR)/epub
 	@echo
 	@echo "Build finished. The epub file is in $(BUILDDIR)/epub."
 
-latex: venv
+latex: ensure-venv
 	$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
 	@echo
 	@echo "Build finished; the LaTeX files are in $(BUILDDIR)/latex."
 	@echo "Run \`make' in that directory to run these through (pdf)latex" \
 	      "(use \`make latexpdf' here to do that automatically)."
 
-latexpdf: venv
+latexpdf: ensure-venv
 	$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
 	@echo "Running LaTeX files through pdflatex..."
 	make -C $(BUILDDIR)/latex all-pdf
 	@echo "pdflatex finished; the PDF files are in $(BUILDDIR)/latex."
 
-text: venv
+text: ensure-venv
 	$(SPHINXBUILD) -b text $(ALLSPHINXOPTS) $(BUILDDIR)/text
 	@echo
 	@echo "Build finished. The text files are in $(BUILDDIR)/text."
 
-man: venv
+man: ensure-venv
 	$(SPHINXBUILD) -b man $(ALLSPHINXOPTS) $(BUILDDIR)/man
 	@echo
 	@echo "Build finished. The manual pages are in $(BUILDDIR)/man."
 
-changes: venv
+changes: ensure-venv
 	$(SPHINXBUILD) -b changes $(ALLSPHINXOPTS) $(BUILDDIR)/changes
 	@echo
 	@echo "The overview file is in $(BUILDDIR)/changes."
 
-linkcheck: venv
+linkcheck: ensure-venv
 	$(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck
 	@echo
 	@echo "Link check complete; look for any errors in the above output " \
 	      "or in $(BUILDDIR)/linkcheck/output.txt."
 
-doctest: venv
+doctest: ensure-venv
 	$(SPHINXBUILD) -b doctest $(ALLSPHINXOPTS) $(BUILDDIR)/doctest
 	@echo "Testing of doctests in the sources finished, look at the " \
 	      "results in $(BUILDDIR)/doctest/output.txt."
 
 htmlview: html
 	$(PYTHON) -c "import os, webbrowser; webbrowser.open('file://' + os.path.realpath('_build/html/index.html'))"
 
-check: venv
+check: ensure-venv
 	# Ignore the tools and venv dirs and check that the default role is not used.
 	$(SPHINXLINT) -i tools -i $(VENVDIR) --enable default-role