build: Add environment variable to set alternative venv path

Signed-off-by: Sven Sager <akira@narux.de>

Makefile

@@ -5,23 +5,34 @@ MAKEFLAGS     = --no-print-directory --no-builtin-rules
 # Variables
 PACKAGE = revpimodio2
 
-# If virtualenv exists, use it. If not, use PATH to find, except python3
-SYSTEM_PYTHON  = /usr/bin/python3
-PYTHON         = $(or $(wildcard venv/bin/python), $(SYSTEM_PYTHON))
+# Set path to create the virtual environment with package name
+ifdef PYTHON3_VENV
+VENV_PATH = $(PYTHON3_VENV)/$(PACKAGE)
+else
+VENV_PATH = venv
+endif
+
+# If virtualenv exists, use it. If not, use PATH to find commands
+SYSTEM_PYTHON  = python3
+PYTHON         = $(or $(wildcard $(VENV_PATH)/bin/python), $(SYSTEM_PYTHON))
 
 all: build docs
 
 .PHONY: all
 
 ## Environment
+venv-info:
+	echo Using path: "$(VENV_PATH)"
+	exit 0
+
 venv:
-	$(SYSTEM_PYTHON) -m venv venv
-	source venv/bin/activate && \
+	$(SYSTEM_PYTHON) -m venv "$(VENV_PATH)"
+	source $(VENV_PATH)/bin/activate && \
 		python3 -m pip install --upgrade pip && \
 		python3 -m pip install -r requirements.txt
 	exit 0
 
-.PHONY: venv
+.PHONY: venv-info venv
 
 ## Build, install
 build:
@@ -41,6 +52,6 @@ clean:
 	rm -rf build docs/_build dist src/*.egg-info *.spec
 
 clean-all: clean
-	rm -R venv
+	rm -R $(VENV_PATH)
 
 .PHONY: clean clean-all