Many visibility warnings building macOS SDK

[1ec5]

As of 8403883 on master, a clean build of the macOS SDK in Xcode 8.2 results in 136 warnings of the following form:

ld: warning: direct access in function 'mbgl::Tileset const& mapbox::util::variant<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, mbgl::Tileset>::get<mbgl::Tileset, (void*)0>() const' from file '/Users/mxn/hub/mapbox-gl-native/build/macos/Debug/libmbgl-core.a(offline_download.o)' to global weak symbol 'typeinfo for mapbox::util::bad_variant_access' from file '/Users/mxn/hub/mapbox-gl-native/build/macos/macos.build/Debug/dynamic.build/Objects-normal/x86_64/NSArray+MGLAdditions.o' means the weak symbol cannot be overridden at runtime. This was likely caused by different translation units being compiled with different visibility settings.

GCC_SYMBOLS_PRIVATE_EXTERN is YES for the dynamic target in macos.xcodeproj (whereas it’s NO for the dynamic and static targets in ios.xcodeproj), but setting it to NO doesn’t address the issue.

This issue doesn’t reproduce on the release-ios-v3.4.0 branch.

/cc @kkaefer

[kkaefer]

GCC_SYMBOLS_PRIVATE_EXTERN adds -fvisibility=hidden for static libraries/executables, but apparently doesn't do this for dynamic libraries.

[1ec5]

What about the iOS dynamic framework? I haven’t noticed these warnings there.

[kkaefer]

That's because we're not yet compiling iOS with -fvisibility=hidden for the core library. PR for iOS is at #7131