This is a good question because LLVM and CLang and GNUstep don't claim to be hard or complicated to use. They do ask their users to read (and reread) their documentation though. I've just discovered this combination for Ubuntu myself and it is very intriguing. And there are a lot of moving parts at the moment.
If the OP got an answer on one of the gnustep mailing lists, then it would be nice to hear here how it was resolved.
Without suggesting the normal route of starting over and only installing the latest from LLVM and GNUstep directly, here's my take.
The OP is trying to use ARC and blocks with the runtime provided by the gunstep/libobjc2 project.
libobjc2 should probably be built with clang version 3.2 - or maybe the top of tree. clang -v will tell you what version you have on your machine. I've found even Ubuntu 12.10 doesn't provide clang-3.2. So I download it from the LLVM website. They have prebuilt binaries for Ubuntu 12.04 LTS. To get make to use clang for this build step, I've seen instructions to set and export CC=clang and CXX=clang++.
Once libobjc2 is built, care must be taken where it gets installed. If the system already has an older libobjc.so.x.y library (this provides the runtime environment for Obj-C), clang or the linker may be picking up the wrong one when you start compiling your own source. I found libobjc.so.4.6.0 built from the libobjc2 source was installed to /usr/local/lib both on Ubuntu 12.04 and 12.10 when I ran the make install step. This path was not setup by the libobjc2 install step.
To get this library picked up, I had to add /usr/local/lib to the env variable LD_LIBRARY_PATH. It wouldn't hurt to try 'locate libojbc.so' to see if there are other versions on the system.
Finally, the libobjc2 readme, https://github.com/gnustep/gnustep-libobjc2#readme, says this new version of the gnustep library supports two ABIs, and each ABI supports ARC and blocks. The advice I've seen about compiling with libobjc2 is to also provide the runtime version expected to be compiling and linking with: -fobjc-runtime=gnustep. Clang on Ubuntu still defaults to the older ABI. -fobjc-nonfragile-abi may accomplish the same thing but I've seen it mentioned that flag was or will be deprecated.
The OP error may actually come from the step where a.out is being run, rather than where clang was compiled. Perhaps the dynamic loader is picking up the legacy libobj.so. Again, I would use locate to see if the system even has more than one.