Checking the ccmake settings for libobjc2, it is already setup to use libsupc++.
Here is the ninja build failure.
/usr/local/bin/clang
-std=gnu99 -fexceptions
Test/CMakeFiles/AllocatePair_optimised.dir/AllocatePair.m.o
-o Test/AllocatePair_optimised
libobjc.so.4.6 -lsupc++ -Wl,-rpath,/home/frank/objc/build-1.7-RC1
libobjc.so.4.6: undefined reference to `vtable for std::type_info'
libobjc.so.4.6: undefined reference to `std::type_info::__is_function_p() const'
libobjc.so.4.6: undefined reference to `typeinfo for std::type_info'
libobjc.so.4.6: undefined reference to `std::type_info::~type_info()'
libobjc.so.4.6: undefined reference to `std::type_info::__do_upcast(__cxxabiv1::__class_type_inf
o const*, void**) const'
libobjc.so.4.6: undefined reference to `std::type_info::__is_pointer_p() const'
clang-3.4: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.