Suitable clang stdenv for Darwin

Doesn't involve GCC at any point. This will be necessary once 10.10 comes around.
I had to add `-idirafter $SDKROOT/usr/include/c++/4.2.1` to `NIX_CFLAGS_COMPILE` in order to get clang to build anything using the c++ standard libraries, but I have no clue if this is the right place to do it.