popt.h does not contain a extern "C" clause to prevent function mangling when using a C++ compiler. This causes the symbols in libpopt to not be found by g++ when compiling.
This has been fixed in (at least) Red Hat 6.0 popt-1.3-1 and in the popt CVS repository. Recompile and install the popt-1.3 from Red Hat 6.0.