These values can be overridden with some poorly documented and
overloaded options: --libc and --sdk-path

../libvpx/configure --target=armv7-darwin-gcc --sdk-path=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer --libc=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk/

So for someone who still wants to build with the iOS 5 SDK, the last
part of the path should be iPhoneOS5.0.sdk

Change-Id: Ibe93d96ae828c619700dc3222983aa4c30456b88