According you error, it should be like you're trying to link a libc++ with stdlibc++, the libc++ and stdlibc++ is different, stdlibc++ is gcc's c++ standard library, it will not compatible with each other.
For your issue, it's like your libpoppler.so is using stdlibc++, but in your clang command line, you're trying use libc++ as standard lib, they have different name in linking stage, see link at the end of this answer for detail why.
So, maybe your solution is just change the compile command to
clang++ -stdlib=libstdc++ po.cxx -lpoppler
Please see this question for detail why std:__1::set and std::set.
Why can't clang with libc++ in c++0x mode link this boost::program_options example?