Compiler baselines

[Stephan Bergmann]

On 23/11/2018 00:48, Thorsten Wagner wrote:
> Due to other changes building master is no longer possible with macOS 10.11. SDK 10.13 is required now (maybe SDK 10.12 but I’m unable to verify this).

According to README.md (in the root of the core repo), our build 
baseline is 10.12, while the runtime baseline we target with those 
builds is 10.9.

(And I'm not going to change any of that in the context of this compiler 
baseline bump.)