Allow C99 extensions in iOS and OS X SDK targets

[1ec5]

C99 extensions are currently disallowed throughout the project due to pedantic warnings being enabled. This makes no sense in Objective-C code, since we’ve been able to depend on C99 support for years. At the least, the iOS and OS X SDK targets should get a -Wno-c99-extensions flag. This will enable us to remove a fair amount of redundant code in MGLMapView.

If any non-SDK Objective-C code needs to use C99-specific features like designated initializers or the Elvis operator, we can selectively turn off the Clang warning via #pragma clang diagnostic ignored "-Wc99-extensions" on a case-by-case basis.

/cc @jfirebaugh @boundsj

[boundsj]

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