Fixed -U undefine flag being dropped when separated from its name b…

[Isl-n]

…y a space // Resolve #5237

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[ivankravets]

How did you find PlatformIO?

[ivankravets]

Thanks for the PR. Resolved in #5237

[Isl-n]

I found PlatformIO while working on an ESP32 project at a course and really liked the ecosystem. Thanks for building it—I'll keep an eye out for other open issues to contribute to!

[ivankravets]

I like the solution in this PR. Did you use any AI model?

[Isl-n]

Yes, Here is my exact workflow:
1- Used AI code review and search tools to scan the repository for open bugs/features to work on.
2- Selected this issue and worked through the fix using Claude Sonnet.
3- Ran pytest locally to verify the solution and make sure all tests passed green.
4- Submitted the PR once everything was fully tested!