[CMake] Generate Visual Studio 2017 project with Basic Runtime Checks off

[James Swift]

Hi,

I'm trying to generate a VS project where Basic Runtime Checks `/RTC1`
is not set for a file in a debug build.

with this minimal example

```
project(testrtc)
add_executable(testrtc testrtc.c)
```

`cmake . -G "Visual Studio 15 2017"`

the switch `/RTC1` is set for the Debug configuration.

Is there a way to force it off?

thanks,
James