mention pythia8 needs to be >= 8180.
port of v5-34-00-patches to support libc++.

This port is needed as OS X 10.9 will support natively only libc++
as C++ run-time library. Extra complication is the needed hand tuning of the
changed libc++ STL and iostream implementation for CINT. Some CINT STL
interfaces still need to be fixed before this port is operational.
Compile using --enable-cxx11 (which automatically turns on --enable-libcxx).