An Objective-C implementation comes in two components; a compiler and a runtime library.
If you are using a GCC 4.4.2 to compile objc code then you should use a GCC 4.4.2 of the Objective-C runtime library 
that came with compiler.
Currently, no features of Objective-C 2 work with GCC. 

But if we want to compile Objective-C 2 programm you should use runtime libobjc2 from GNUstep or Étoilé's ObjectiveC2 
framework.
For more advanced features, currently only supported if you compile with Clang, you will need the GNUstep runtime.
The GNUstep runtime supports two ABIs. One is compatible with the GCC runtime, the other is new and is designed to 
support Objective-C 2.
Clang is a new front end for LLVM( Low-Level Virtual Machine ). 
If you compile with Clang then you have access to declared properties, fast enumeration, and blocks. 
If you compile with the -fobjc-nonfragile-abi flag then you will use the new ABI. This is only supported with the 
GNUstep runtime. This adds: 

1. Property introspection 
2. Introspection on optional protocol methods 
3. Non-fragile instance variables 
4. Forwarding proxy support 

I compiled on QNX 6.4.1 libobjc2 from GNUstep how also LLVM and Clang.
Clang compiles both programmes in C and objc.
It for compilation objc was one should add ADDITIONAL_CFLAGS = -D__QNXNTO__ -D__X86__ -D__LITTLEENDIAN__ in GNUmakefile 
or 
to use:
clang -D__QNXNTO__ -D__X86__ -D__LITTLEENDIAN__  prg.m -L / usr / pkg / lib -lobjc 
Catalogue / usr / pkg / lib is to runtime  path libobjc2 from GNUstep - libobjc.so.4 and clang will use this library and
 not GCC runtime.

I compiled on QNX 6.4.1 libobjc2 from GNUstep how also LLVM and clang.
clang compiles both programmes in C and objc.
For compilation objc we should add ADDITIONAL_CFLAGS = -D__QNXNTO__ -D__X86__ -D__LITTLEENDIAN__ in GNUmakefile or 
to use:
clang -D__QNXNTO__ -D__X86__ -D__LITTLEENDIAN__  prg.m -L / usr / pkg / lib -lobjc 
/usr/pkg/lib is directory to runtime libobjc2 from GNUstep - libobjc.so.4 and clang will use this library and not GCC 
runtime.

I do not know or someone this will interest but on http://www.ajam.org.pl I will place the description of how to compile
 LLVM, clang and libobjc2 tomorrow.