Auto merge of #14346 - mati865:master, r=UK992,larsberg

Use MSVC toolchain if PLATFORM is defined

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`VSInstallDir` exists only when Visual Studio is installed.
`VS140COMNTOOLS` is defined by installing either Visual Studio or [Visual C++ Build Tools](http://landinghub.visualstudio.com/visual-cpp-build-tools)

It will allow to build MSVC based Servo without having to install whole Visual Studio (few GiB).

EDIT: Another (maybe cleaner) solution would be reverting https://github.com/servo/servo/commit/63ec96a57d4772a961bea63111c927ccd7c85a91

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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because `VS140COMNTOOLS` variable is already used in [mach.bat](https://github.com/servo/servo/blob/master/mach.bat)

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