[swift-evolution] Renaming "Build Configuration"

[Jordan Rose]

> 
> On Jan 28, 2016, at 11:08, Kevin Ballard via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Renaming this seems like a good idea.
>  
> On Wed, Jan 27, 2016, at 05:18 PM, Jordan Rose via swift-evolution wrote:
>> - "Custom Compilation Features" for all the user-defined options.
>> - "marks a custom compilation feature as present" as the --help text for -D.
>>  
>> …but I'd be happy if someone comes up with something better. (And I'd like feedback on if any of these names are problematic in some other way.)
>  
> How about "Conditional Compilation Flags" for the user-defined options? The term "flag" implies boolean, and also connotes the presence of the flag as being the true value. The help text can then say "marks a conditional compilation flag as true". The boolean nature of the flag also makes the syntax `#if SomeFlag` make more sense, because we're used to if conditions being boolean values.
>  

Closing the loop: I implemented this in https://github.com/apple/swift/commit/c90511a74e94a06f1199ecd3f8d004cbfa70410d, taking Kevin's suggestion of "flags" for the user-set options. Jack L from Apple convinced me to avoid the word "feature" as well, since the platform conditions refer to mutually exclusive environments ("os(OSX)" vs. "os(iOS)"), while "features" can usually be mixed.

Jordan