How to install clang in custom location from SVN, making it recognize c++ standard library, on OS X - c++

Question

I try to compile and use clang from svn trunk. I basically try to follow the directions here:
svn co -q http://llvm.org/svn/llvm-project/llvm/trunk llvm
svn co -q http://llvm.org/svn/llvm-project/cfe/trunk llvm/tools/clang
svn co -q http://llvm.org/svn/llvm-project/clang-tools-extra/trunk llvm/tools/clang/tools/extra
svn co -q http://llvm.org/svn/llvm-project/compiler-rt/trunk llvm/projects/compiler-rt
mkdir llvm_build_release
cd llvm_build_release
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=$HOME/usr/local -DLLVM_TARGETS_TO_BUILD=host ../llvm
make -j12
make install
Above, I configure clang to be installed in the custom location ~/usr/local since I want to be able to play with it without changing my default environment.
I then create a simple test.cpp:
#include <iostream>
int main(int argc, const char* argv[]){
std::cout << "Hello world\n";
return 0;
}
and try to compile it:
~/usr/local/bin/clang++ test.cpp -o test
but get the error message:
test.cpp:1:10: fatal error: 'iostream' file not found
#include <iostream>
^
1 error generated.
(using the system version of clang, the same compilation works fine).
If I manually enter which standard library to use, it does work
~/usr/local/bin/clang++ -std=c++11 -stdlib=libstdc++ -I/usr/include/c++/4.2.1/ -L/usr/lib test.cpp -o test
The question is: How do I configure, compile and install clang from source so that I do not have to enter these standard library settings, but instead can just enter the ordinary ~/usr/local/bin/clang++ test.cpp -o test? I have macports installed, with its version of the standard libraries and the include files, if that helps.