coreutils: disallow on-device build

It can delete utility 'ln' in the process of installation:

 make  install-exec-hook
 make[4]: Entering directory '/data/data/com.termux/files/home/.termux-build/coreutils/build'
 /bin/sh: 12: ln: not found

packages/coreutils/build.sh

@@ -27,4 +27,10 @@ ac_cv_func_getpass=yes
 termux_step_pre_configure() {
 	CPPFLAGS+=" -DDEFAULT_TMPDIR=\\\"$TERMUX_PREFIX/tmp\\\""
 	CPPFLAGS+=" -D__USE_FORTIFY_LEVEL=0"
+
+	# On device build is unsupported as it removes utility 'ln' (and maybe
+	# something else) in the installation process.
+	if [ -n "$TERMUX_ON_DEVICE_BUILD" ]; then
+		termux_error_exit "Package '$TERMUX_PKG_NAME' is not safe for on-device builds."
+	fi
 }