Add /WX (warnings as errors) compile flag when compiling with MSVC.

[heijp06]

This PR fixes #922. When compiling with MSVC (Microsoft Visual C++) warnings are now treated as errors for all exercises.

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[heijp06]

@vaeng, I created this PR for #922.

[vaeng]

I assume you replaced the lines for all exercises and did not add anything else? I scrolled through the changes and read your explanation in the issue. Thanks for the write-up that helps a lot to speed things up.

I would love to have a single space for those files and have each exercise pull from that folder, but currently, we have to change it everywhere.

It is a good change, if all tests pass, we should merge this.

[vaeng]

I see you did some other corrections. It's good that you kept those in different commits, so I can judge them faster.