Reduce compilation time

[friedbunny]

We should investigate ways to reduce compilation times.

Anecdotally, compilation time has been steadily creeping up — it currently takes ~10 minutes for a clean iOS debug build in Xcode to complete on my MacBook Pro (Retina, 13-inch, Early 2015, 3.1 Ghz). This is quite long and negatively affects productivity.

Breakdown by target, in the order they’re built when running ios-app:

🎯	⏱
mbgl-core	7 minutes
dynamic	2 minutes
ios-app	1 minute

/cc @mapbox/gl

[1ec5]

If you see any opportunities to replace Objective-C++ files with pure Objective-C files in the dynamic target, we should look into doing so. Objective-C++ is known to be slow to compile due to those pluses.

[friedbunny]

This has been and is being addressed in several places:

• Update CircleCI images and add Address/Undefined Behavior sanitizers #9399 — Enable ccache
• Only build active architecture in Xcode #7388 — Only build active architecture when building Debug builds to speed up compile time
• Update compile flags for iOS #7131 — Update compile flags for iOS
• Switch to Xcode 9 — waiting on its release this fall.
• Get a computer with more cores. 😬

[friedbunny]

I think we’re in a decent place with ccache and other incremental improvements — nothing actionable left here.