Make sure _LIBCPP_EXTERN_TEMPLATE is not defined (used by libc++).

The default definition of this macro causes extern template
symbols to be undefined when running with cling. It has been fixed in
the meanwhile in the libc++ sources but the libc++ that ships with OSX 10.9
still has this macro defined. For the discussion on this see:
http://clang-developers.42468.n3.nabble.com/LIBCPP-INLINE-VISIBILITY-and-std-string-length-td4030879.html
and
http://www.mail-archive.com/cfe-commits@cs.uiuc.edu/msg79202.html