make osx builds OS X framework without Xcode, fails

[1ec5]

make osx attempts to build the OS X framework using make instead of xcodebuild. The framework target relies on Xcode-specific build settings, so it fails to build under make:

../../platform/osx/sdk/Mapbox.m:1:9: fatal error: 'Mapbox/Mapbox.h' file not found
#import <Mapbox/Mapbox.h>
        ^
1 error generated.

If make osx is useful in its own right, it should avoid building the framework and osxapp and limit itself to just the headless parts. Otherwise, we should remove the target because we already have make ipackage and make xosx.

/cc @brunoabinader @kkaefer

[kkaefer]

It's fine to remove that now that we have made the pact with the devil^H^H^H use Xcode.

[1ec5]

Hey now. 😉

[brunoabinader]

We're handling make linux build on OSX in #3294.