Allow building other targets without Python3 and virtualenv

TXT2RST       = $(VENV)/bin/txt2rst

PYTHON        = $(shell which python3)

HAS_PYTHON3    = NO

HAS_VIRTUALENV = NO

ifeq ($(shell which python3 >/dev/null 2>&1; echo $$?), 1)

$(error Python3 was not found! Please check README.md for further instructions)

ifeq ($(shell which python3 >/dev/null 2>&1; echo $$?), 0)

HAS_PYTHON3 = YES

endif

ifeq ($(shell which virtualenv >/dev/null 2>&1; echo $$?), 1)

$(error virtualenv was not found! Please check README.md for further instructions)

ifeq ($(shell which virtualenv >/dev/null 2>&1; echo $$?), 0)

HAS_VIRTUALENV = YES

endif

SOURCES=$(wildcard src/*.txt)

	)

$(VENV):

	@if [ "$(HAS_PYTHON3)" == "NO" ] ; then echo "Python3 was not found! Please check README.md for further instructions" 1>&2; exit 1; fi

	@if [ "$(HAS_VIRTUALENV)" == "NO" ] ; then echo "virtualenv was not found! Please check README.md for further instructions" 1>&2; exit 1; fi

	@( \

		virtualenv -p $(PYTHON) $(VENV); \

		. $(VENV)/bin/activate; \