win64: build C files with /GS- to avoid "security" calls

[rainers]

This avoids some symbol references into the VC compiler runtime and makes it easier to replace it with another runtime.
I suspect there are no similar security checks on other platforms.

[dlang-bot]

Thanks for your pull request, @rainers!

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[MartinNowak]

I'm somewhat uneasy about this @rainers, any chance we could stub the necessary methods in toolchains that lack them?
Disabling protections in a library that process user input (zlib) and might not get updated regularly sounds like a bad idea (thx to Iain for the recent updates, https://github.com/dlang/phobos/commits/master/etc/c/zlib/README).
https://www.zlib.net/ChangeLog.txt
-GS (Buffer Security Check)

[rainers]

@MartinNowak I don't think we need to be very pessimistic here. zlib is a pretty well tested library. I suspect there are no such tests on any other platform.