Include application and operating system name and version in user agent string

[1ec5]

The user agent string for all non-telemetry requests in the iOS and OS X SDKs now conforms to the following format:

MyApp/0.1.2 Mapbox/3.4.5 MapboxGL/6.7.8 (deadfeed) iOS Simulator/9.0.1 (x86_64)
^ app       ^ sdk        ^ core                    ^ os                 ^ arch

This file only links against the cross-platform CoreFoundation and Foundation frameworks, not the platform-specific AppKit or UIKit frameworks, so the options for identifying the operating system and device are fairly limited. (I didn’t want to go through such great lengths to include all those details.)

Fixes #3997.

@kkaefer for review. @boundsj, can you confirm that this beefed-up user agent string will be compatible with the style API and dashboard?

[kkaefer]

Can we use TargetConditionals.h to know more about the platform we're building for? So instead of Darwin, we'd use iOS, iOS Simulator, or Mac OS X.

[kkaefer]

Ah sorry, the last commit does that.

Can we cache the value of the the User Agent string composition?

[1ec5]

@kkaefer, do you know why the version.hpp include would fail only on clean builds for iOS (but not OS X)?

[mikemorris]

do you know why the version.hpp include would fail only on clean builds for iOS (but not OS X)?

Because it's generated and the iOS build might be missing this step? https://github.com/mapbox/mapbox-gl-native/blob/master/gyp/version.gypi

[kkaefer]

since we can detect it from the architecture

what are we going to do when Apple introduces an x86_64 iOS device?

[1ec5]

what are we going to do when Apple introduces an x86_64 iOS device?

We’re going to build an SDK for it and update this logic. 😄

[1ec5]

Because it's generated and the iOS build might be missing this step? https://github.com/mapbox/mapbox-gl-native/blob/master/gyp/version.gypi

@mikemorris, including a version dependency should be enough, right?