Libc++ can be build against many different C++ ABI libraries (see http://libcxx.llvm.org/). Therefore qbs should either determine the ABI library itself or offer configuration.
As a workaround I currently set "cpp.cxxStandardLibrary: undefined" and add the needed flags myself to cxxFlags and linkerFlags. While this seems to compile and link fine, qbs prints this warning: "WARNING: Conflicting scalar values" when using other modules that use "cxxStandardLibrary" like qt.
If selection of the ABI library can not be automated then I think you should add a "cxxAbiLibrary" property similar to "cxxStandardLibrary". Im currently using this: