Add support out-of-tree builds. This commit adds a support for out-of-tree builds, i.e, object files, executables and so on are writen to `$(OUTDIR)` (which defaults to `build/<BUILD_TARGET>` directory. This allows building Smalltalk/X from single source tree for multiple targets.

#!/bin/bash
set -e

test -f .Xauthority.org && xauth -f .Xauthority.org list | sed -e '/unix/!d' -e "s+.*/unix:+add $(hostname)/unix:+" | xauth 2>/dev/null -
test -z "$1" && exec stx/projects/smalltalk/stx
exec "$@"