Update of threading documentation in chapter 23

[Gabriel Dos Reis]

Loren James Rittle <rittle@latour.rsch.comm.mot.com> writes:

|    <p><em>However, please ignore all discussions about the user-level
|       configuration of the lock implementation inside the STL
|       container-memory allocator on those pages.  For the sake of this
|       discussion, libstdc++-v3 configures the SGI STL implementation,
|       not you.  This is quite different from how gcc pre-3.0 worked.
|       In particular, past advice was for people using g++ to
|       explicitly define _PTHREADS or other macros or port-specific
|       options on the command line to get a thread-safe STL.  This is
|       no longer required for any port and should no longer be done
|       unless you really know what you are doing and assume all
|       responsibility.</em>

Well, we should then take the opportunity to document g++ command line
options (-pthreads or -pthread, ...) needed to activate MT in the
library and linking process.

Thanks,

-- Gaby