better handling of installation when virtualenv not present

Makefile

@@ -0,0 +1,7 @@
+all: build
+
+build:
+	bash build.sh
+
+clean:
+	rm -rf ./*.egg-info ./__pycache__/ ./*/__pycache__/ ./venv/

build.sh

@@ -12,9 +12,40 @@ elif which python >/dev/null; then
     fi
 else
     echo "Could not find python >= 3 in PATH"
+    exit 1
 fi
 
+# try to set up virtualenv
 $pyexec -m virtualenv venv
-[[ -f ./venv/bin/pip ]] \
-    && ./venv/bin/pip install --editable . \
-    || ./venv/Scripts/pip install --editable .
+
+# venv not created
+if [ ! -d ./venv ]; then
+    $pyexec -m pip install --user virtualenv
+    $pyexec -m virtualenv venv
+
+    if [ ! -d ./venv ]; then
+        echo "ERROR: Unable to create python virtualenv"
+        exit 1
+    fi
+fi
+
+if [ -f ./venv/bin/pip ]; then
+    pipexec=./venv/bin/pip
+    activateexec=./venv/bin/activate
+elif [ -f ./venv/Scripts/pip ]; then
+    pipexec=./venv/Scripts/pip
+    activateexec=./venv/Scripts/activate
+else
+    echo "ERROR: Unable to find python pip in virtualenv"
+    exit 1
+fi
+
+$pipexec install --editable .
+
+echo
+if [ -f $activateexec ]; then
+    echo "Installed. Activate environment with: source $activateexec"
+else
+    echo "ERROR: Unable to find virtualenv activate script"
+    exit 1
+fi