Clean up iOS.cmake to use xcrun/xcodebuild & libtool.

- Substantial cleanup of iOS.cmake to use xcrun & xcodebuild to
  determine the SDK & tool paths.
- Use libtool -static to link libraries instead of ar + ranlib, which
  is not compatible with Xcode 7+, this change should be backwards
  compatible to at least Xcode 6.
- Force locations of unordered_map & shared_ptr on iOS to work around
  check_cxx_source_compiles() running in a forked CMake instance without
  access to the variables (IOS_PLATFORM) defined by the user.
- Minor CMake style updates.

Change-Id: I5f83a60607db34d461ebe85f9dce861f53d98277