[mod] more robust make pyenv / make pyenvinstall

"make pyenv" ensures that ./local/py3/bin/python is an executable

utils/makefile.python

@@ -99,7 +99,8 @@ quiet_cmd_pyinstall = INSTALL   $2
 quiet_cmd_pyenvinstall = PYENV     install $2
       cmd_pyenvinstall = \
 	if ! cat $(PY_ENV)/requirements.sha256 2>/dev/null | sha256sum --check --status 2>/dev/null; then \
-		$(PY_ENV_BIN)/python -m pip $(PIP_VERBOSE) install -e $2$(PY_SETUP_EXTRAS) ;\
+		rm -f $(PY_ENV)/requirements.sha256; \
+		$(PY_ENV_BIN)/python -m pip $(PIP_VERBOSE) install -e $2$(PY_SETUP_EXTRAS) &&\
 		sha256sum requirements*.txt > $(PY_ENV)/requirements.sha256 ;\
 	else \
 		echo "PYENV     $2 already installed"; \
@@ -119,13 +120,13 @@ quiet_cmd_pyenvuninstall = PYENV     uninstall   $2
 # $2 path to folder where virtualenv take place
 quiet_cmd_virtualenv  = PYENV     usage: $ source ./$@/bin/activate
       cmd_virtualenv  = \
-	if [ ! -d "./$(PY_ENV)" ];then                                  \
-		$(PYTHON) -m venv $(VTENV_OPTS) $2;                     \
+	if [ -d "./$(PY_ENV)" -a -x "./$(PY_ENV_BIN)/python" ]; then \
+		echo "PYENV     using virtualenv from $2"; \
+	else \
+		$(PYTHON) -m venv $(VTENV_OPTS) $2; \
 		$(PY_ENV_BIN)/python -m pip install $(PIP_VERBOSE) -U pip wheel setuptools; \
-		$(PY_ENV_BIN)/python -m pip install $(PIP_VERBOSE) -r requirements.txt;     \
-	else                                                            \
-		echo "PYENV     using virtualenv from $2";              \
-        fi
+		$(PY_ENV_BIN)/python -m pip install $(PIP_VERBOSE) -r requirements.txt; \
+	fi
 
 # $2 path to lint
 quiet_cmd_pylint      = LINT      $@