Added support for generated_file targets and several bug fixes for iOS

- Reenabled Python on iOS builds (example usage: automatic generation of resources - icons...)
- Bug fix in link_framework (if we directly edit LINK_FLAGS the -framework ... flags are not transitively transfered to dependencies
- Added -all_load flag for Objective-C to preserve category-only symbols
- Added SSToolkit library, need to figure out how to specify its framework dependencies as well.