Add SWIFT_PLATFORMS_USING_ISYSROOT to list platforms that should be building with -isysroot

[kubamracek]

No description provided.

[compnerd]

Hmm, I'm not sure I understand this. If I'm working with freestanding, I could be on a Linux or windows environment, which don't use isysroot. I realize that those configurations aren't in CI currently, but this configuration makes that more challenging. Maybe we should have a prefer isysroot option instead that's false and true on Darwin hosts by default but the user can change?

[kubamracek]

You're right. (this is not a well-thought-out PR, I just wanted to see your take on this.) We should make the freestanding config be usable from any environment. My immediate problem is when building freestanding on a macOS host, I need to use -isysroot (that's what all Apple SDKs expect). Let me try adding this as an option into the SDK configuration itself.

[kubamracek]

@compnerd Updated. Notice that currently the FREESTANDING SDK is set up as configure_sdk_darwin, so it does get USE_ISYSROOT=1. If we wanted to actually start building FREESTANDING on Linux, we'd have to add a configure_sdk_unix somewhere.

[kubamracek]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 046a174

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 046a174

[compnerd]

Hah! I think you put this in the wrong commit 🤷‍♂️

[kubamracek]

No, this was an intentional drive-by addition :) It seemed to be missing...