Don't recommend building Qt for iOS as -developer-build

It enables -Werror and other "goodies" that we don't want to bother
end users building Qt from source with.

Change-Id: I36e7c8e3f8af3b7907b515648ff65ff83574bffb
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>