When developing for iOS the default value in XCode for Build Settings > Apple LLVM 5.0 - Language - C++ > C++ Standard Library is libstdc++ (GNU C++). It seems that this cannot be changed. I would like to use 3rd party library which uses libc++ so is there some workaround available?
no,
for backwards compatibility reasons we use libstdc++ internally for now
so you wont be able to link unity with clang stdlib