Prevent trying to run on-device (fixes #1910)

README.md

@@ -3,8 +3,7 @@ Termux packages
 [![Build Status](https://travis-ci.org/termux/termux-packages.svg?branch=master)](https://travis-ci.org/termux/termux-packages)
 [![Join the chat at https://gitter.im/termux/termux](https://badges.gitter.im/termux/termux.svg)](https://gitter.im/termux/termux)
 
-This project contains scripts and patches to build packages for the
-[Termux](https://termux.com/) Android application.
+This project contains scripts and patches to build packages for the [Termux](https://termux.com/) Android application. Note that packages are cross compiled and that on-device builds are not currently supported.
 
 Setting up a build environment using Docker
 ===========================================

build-package.sh

@@ -8,6 +8,10 @@ termux_error_exit() {
 	exit 1
 }
 
+if [ `uname -o` = Android ]; then
+	termux_error_exit "On-device builds are not supported - see README.md"
+fi
+
 # Utility function to download a resource, optionally checking against a checksum.
 termux_download() {
 	local URL="$1"