Merge: iOS: fix compilation problems, warnings and the Benitlux ios-release rule

General fixes for the iOS support:
* Report errors on `rmdir`, this depends on the fix #2309.
* Fix release mode compilation, the path to the binary was wrong.
* Fix the ios-release rule of Benitlux to use the release mode, contact the right server, and keep the generated files to use from XCode.
* Finally silence the normal output of `xcodebuild`. This looks like a new feature of XCode 8!
* Fixed a few warnings raised by clang.

Pull-Request: #2316
Reviewed-by: Jean Privat <jean@pryen.org>