Add Makefile to update requirements files

# Makefile for updating BuildStream's requirements files.

#

REQUIREMENTS_IN := $(wildcard *.in)

REQUIREMENTS_TXT := $(REQUIREMENTS_IN:.in=.txt)

PYTHON := python3

VENV := $(PYTHON) -m venv

VENV_PIP = $(VENVDIR)/bin/pip

.PHONY: all

all: $(REQUIREMENTS_TXT)

%.txt: %.in

	$(eval VENVDIR := $(shell mktemp -d $$TMPDIR/bst-venv.XXXXXX))

	$(VENV) $(VENVDIR)

	$(VENV_PIP) install -r $^

	$(VENV_PIP) freeze -r $^ > $@

	rm -rf $(VENVDIR)