c11 is being used instead of gnu17

[sean-mcmanus]

See the repro at microsoft/vscode-cpptools#6279 .

[andreeis]

Possibly related #1489.

[Colengms]

This issue is due to falling back to a hard-coded default, and not querying the compiler to determine what its default actually is. To avoid requiring all config providers from having to do this, we've made a change to the cpptools-api to allow IntelliSenseMode, cStandard and cppStandard to be omitted, and automatically determined by cpptools based on the compilerPath and compilerArgs. I think we have some other changes we'd like make to the API before publishing a new version. Once a new version of cpptools-api is published, CMake Tools can be updated to omit these fields.