[iOS] Replaced deprecated podspec flag

Per #1870 (comment).

ref CocoaPods/CocoaPods#3573

CHANGELOG.md

@@ -23,6 +23,7 @@ Known issues:
 
 ## iOS master
 
+- If you install this SDK via CocoaPods, CocoaPods version 0.38.0 or above is required. ([#2132](https://github.com/mapbox/mapbox-gl-native/pull/2132))
 - The `styleID` property has been removed from MGLMapView. Instead, set the `styleURL` property to an NSURL in the form `mapbox://styles/STYLE_ID`. If you previously set the style ID in Interface Builder’s Attributes inspector, delete the `styleID` entry from the User Defined Runtime Attributes section of the Identity inspector, then set the new “Style URL” inspectable to a value in the form `mapbox://styles/STYLE_ID`. ([#2632](https://github.com/mapbox/mapbox-gl-native/pull/2632))
 - Default styles such as Streets are no longer bundled with the SDK; instead, they are loaded at runtime from the style API on mapbox.com. As always, you can use these default styles with any valid access token, and Streets continues to be `MGLMapView`’s initial style. The `bundledStyleURLs` property on `MGLMapView` has been deprecated in favor of several class methods on `MGLStyle` that provide direct access to the default styles. ([#2746](https://github.com/mapbox/mapbox-gl-native/pull/2746))
 - The SDK now builds with Bitcode enabled. ([#2332](https://github.com/mapbox/mapbox-gl-native/issues/2332))

ios/Mapbox-iOS-SDK.podspec

@@ -30,7 +30,7 @@ Pod::Spec.new do |m|
 
   m.frameworks = 'CoreLocation', 'GLKit', 'ImageIO', 'MobileCoreServices', 'QuartzCore', 'SystemConfiguration'
   m.libraries = 'c++', 'sqlite3', 'z'
-  m.xcconfig = {
+  m.pod_target_xcconfig = {
     'OTHER_CPLUSPLUSFLAGS' => '-std=gnu++11 -stdlib=libc++',
     'OTHER_LDFLAGS' => '-ObjC',
   }