add missing (though unimportant currently) cxx11 wrapper file
thread_local checking.

Clang (3.2) on OS X 10.6 does not support thread-local storage
(macports GCC does by calling pthreads, and 10.7 implements it
natively).  So auto-detect thread-local storage.  Currently only
one thread uses borrowed_bitset at once so this is a
currently-acceptable workaround.