Add -stdlib=c++ to the LDFLAGS when configuring llvm.  The lldb
xcode project file sets the MACOSX_DEPLOYMENT_TARGET to 10.7.
llvm/configure (as of r199313) tries to compile a .cpp file which
includes <atomic> with -std=c++0x.  If the deployment target is set
to earlier than 10.9 and no C++ library is specified, clang will
error out at this point.

Fixes xcode building of lldb on Mac OS X.  If this change causes
problems for any of the other build workflows, please let me know
and I'll fix it.