configure: quote generated shell environment values

[moreaki]

Summary

Quote non-Windows generated env.sh values before writing export KEY=value lines.

This keeps source ./env.sh working when inherited environment values contain spaces, for example a PATH entry under a macOS .app bundle. Windows env.bat output is unchanged.

The formatting helper converts the value to a string before passing it to shlex.quote, so scalar environment values remain robust as well.

Reproduction

Without quoting, a generated line such as:

export PATH=/usr/bin:/Applications/Little Snitch.app/Contents/Components:/bin

is parsed as a split assignment and an additional invalid export argument.

Validation

• Rebased and applied independently to current origin/main (5ac81eda).
• One file; git diff --check and python3 -m py_compile configure.py pass.
• Ran a clean Darwin configure with the inherited PATH containing /Applications/Little Snitch.app/Contents/Components.
• The generated line is shell-quoted; sourcing env.sh succeeds and restores the exact original PATH, including the space-containing entry.
• The integrated full Darwin/Arm build completed with exit 0 after sourcing that environment and supplying the separate kBuild SDK/tool overrides required by the current tree.

Scope

This PR fixes shell serialization only. It does not claim that all configure SDK selections are propagated into AutoConfig.kmk; the current local Darwin build still requires explicit PATH_SDK_MACOSX* and tool overrides at kmk invocation time.

Closes #750.

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