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.

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#!stx --script

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"/

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"/ notice that you can open more tools from the workspace

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"/ (i.e. eval: NewLauncher open)

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"/

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Workspace open.

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'done' printCR.