libvpx

iOS build configuration updates.

Commit message longer than commit edition.

Simulator and devices:
  Add a common minimum iOS version that can be shared by iOS and iOS
  simulator targets.

Fix --enable-debug (for device targets; sim was fine):
  Allow for successful configuration and build with --enable-debug when
  CXX is available by:

  - Using CXX as LD (when CXX is available).
  - Passing the correct form of the iOS minimum version parameter based on
    whether LD is CXX or really is ld.

  Note: ld -g still won't work on macosx with this patch, so if CXX is not
  available, configuration will still fail reporting that the toolchain
  cannot link executables when attempting to pass --enable-debug (because
  ld returns an error code since the one included with xcode doesn't
  support the -g argument).

Change-Id: Ia488aed167cc2ca82ee9e980589fb76dddce634f