The Cocotron Developer Tools (CDT) for Xcode
The Cocotron Developer Tools are a collection of well known open source projects working together as a cross-compilation system for Xcode.
GNU binutils - linker and object code utilities. ld is modifed to support frameworks via the -F and -framework options.
GNU gcc - Objective-C compiler, modified to support frameworks and The Cocotron Objective-C runtime API.
MinGW - Platform interface for Windows, consists of libraries, headers and program initialization.
Linux, Solaris and possibly other platforms are supported by the creation of a platform interface as described here.