switch to value based settings

[samdeane]

Rather than having to write the settings for each tool / compiler into the configuration explicitly, we should specify settings in a more abstract way, and then have a translation layer which processes them to product the right kind of settings for each tool.

Eg a setting: “minimum-platform” : “macos10.12”

Could be translated to: “-Xswiftc” “-target” “-Xswiftc” “x86_64-apple-macosx10.12” for Swift, but to MACOSX_DEPLOYMENT_TARGET = 10.12 for generating an xcconfig file.