How to cross-compile clang/llvm for iOS?

How to cross-compile clang/llvm for iOS? I need to get libclang (.a or .dylib i believe) to use it via C API in my iOS app.

# Get LLVM/Clang

mkdir llvm
curl -O http://llvm.org/releases/3.4/llvm-3.4.src.tar.gz
tar xzfv llvm-3.4.src.tar.gz
cd llvm-3.4/tools/
curl -O http://llvm.org/releases/3.4/clang-3.4.src.tar.gz
tar xzfv clang-3.4.src.tar.gz
mv clang-3.4 clang
cd ..

# Assuming Xcode 5.1 (LLVM 3.5+ requires -stdlib=libc++ as well)

export CC="clang -arch armv7 -mios-version-min=5.0 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS7.1.sdk"
export CXX="clang++ -arch armv7 -mios-version-min=5.0 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS7.1.sdk"

mkdir build
cd build

../configure \
  --prefix=/Users/thomas/tmp/llvm-ios \
  --host=arm-apple-darwin11 \
  --enable-optimized \
  --disable-assertions

unset CC CXX # important! (Otherwise the next step will fail)

make VERBOSE=1 -j...

After a while you will get:

/Users/thomas/tmp/llvm-3.4/lib/Support/Unix/Program.inc:46:10: fatal error: 'crt_externs.h' file not found
#include <crt_externs.h> // _NSGetEnviron
         ^

Comment the header file and hack the call to _NSGetEnviron() out (you'll get this three times)