MacPorts clang not using its own headers

Question

I'm trying to get emscripten to work on OS X 10.8, see this post for some related issues there. Apparently the clang++ version shipped with Xcode is too old, so I got a recent clang 3.7.0 using MacPorts. I even told CMake to use that compiler (passing -DCMAKE_CXX_COMPILER=clang++-mp-3.7 on the command line), but it still fails:

[ 33%] Building CXX object CMakeFiles/optimizer.dir/parser.cpp.o
/opt/local/bin/clang++-mp-3.7    -std=c++11 -fno-exceptions -fno-rtti -O3 -DNDEBUG
  -o CMakeFiles/optimizer.dir/parser.cpp.o
  -c …/emsdk/emscripten/master/tools/optimizer/parser.cpp
In file included from …/emsdk/emscripten/master/tools/optimizer/parser.cpp:2:
In file included from …/emsdk/emscripten/master/tools/optimizer/parser.h:12:
…/emsdk/emscripten/master/tools/optimizer/istring.h:3:10: fatal error: 
      'unordered_set' file not found
#include <unordered_set>
         ^
1 error generated.

I can reproduce that issue by launching the compiler from the command line. In parallel build mode, sometimes it's instead complaining about <cstdint> for optimizer.cpp instead. Both these headers exist in /opt/local/libexec/llvm-3.7/include/c++/v1/.

• What's the canonical way to use the macports-installed version of clang++ including its headers? Do I have to use -I and work out the full path, or is there something shorter?
• Can I safely do so without also switching the runtime library to the one shipped with MacPorts' clang? If not, can I somehow encode the full path of the runtime library into the created binary, either for that single library or using the -rpath argument to ld or some equivalent alternative?
  Update: I get unresolved symbols when I try to link stuff after specifying the include directory manually, and I don't know how to solve that. The libcxx package from MacPorts is empty except for a readme file.

Answer 1

I've solved the original problem by adding CXXFLAGS=--stdlib=libc++ to the environment. Then even the system version of clang will do everything I need. That flag works magic for MacPorts' version of clang as well: specifying that I get a successful build, and I can even verify (using the -E compiler switch) that it's using the headers I mentioned before. I'm still not certain whether there is anything to ensure that the headers match the system's version of libc++, though.