configure: Enable libc++ with LLVM with clang

When clang is enabled, also pass through --enable-libcpp to LLVM's configure
command line to help it pick up the most recent c++ runtime library. This also
changes the mklldeps.py script to pick up on whether LLVM was linked against
stdc++ or c++ based on the --cxxflags that llvm-config prints.

In an ongoing attempt to update LLVM, the bots need to update their C compilers
to something that supports c++11 (LLVM recently switched). The OSX bots are
running Lion (10.7), which only supports up to gcc 4.2 and clang 3.2. Apparently
the libstdc++ is too old (even on the most updated command line tools) for LLVM,
but using libc++ instead appears to work just fine.