MSVC Subsystem Version Floor Prevents Targeting Windows 2000 or Earlier

[realJoshByrnes]

Summary:
Rusticle binaries built with the MSVC toolchain work correctly on Windows NT 4.0 (and likely Windows 2000 - untested). However, the /SUBSYSTEM flag in link.exe enforces a minimum version of Windows XP (5.01), preventing developers from directly targeting older versions like NT 4.0 or Windows 95.

This limitation requires manual patching of the PE header post-build to lower the subsystem version.

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Current Behavior:

• MSVC link.exe restricts /SUBSYSTEM:CONSOLE,<version> to a minimum of 5.01.
• Rusticle MSVC binaries run successfully on NT 4.0 if the subsystem version is manually patched.
• Rusticle GNU binaries run successfully on NT 4.0

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Expected Behavior:
Developers should be able to opt into legacy subsystem versions (e.g. 3.10, 4.00) without manually editing binaries.

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Proposed Solutions:

• Add a build flag or profile setting to enable legacy subsystem targeting.
• Provide a post-build script that patches the PE header to set the desired subsystem version.
• Consider a build wrapper that invokes editbin, llvm-objcopy, or a custom tool to rewrite the header automatically.
• Document subsystem version limitations and compatibility notes for legacy Windows targets.

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Environment:

• Rusticle v1.20.1
• Target: i686-pc-windows-msvc
• Tested on: Windows NT 4.0 (via VirtualBox)

[realJoshByrnes]

This will also help make #2 possible (building for Win95)