add env var check in install script

install.sh

@@ -5,6 +5,12 @@
 VERSION_NUMBER=$(cat "VERSION")
 echo "installing binarcpython version $VERSION_NUMBER"
 
+# check the BINARY_C environment variable is defined
+if [[ -z "${BINARY_C}" ]]; then
+    echo "Error: The BINARY_C environment variable is not defined. This should point to the directory of your binary_c installation. Please set it and try again."
+    exit
+fi
+
 # we can only use python3 and python3, but allow
 # the user to set these in environment variables
 # PYTHON and PIP.