Installation Error

Hi All,
I am installing llvm 3.1 and clang from the source on my Mac os x 10.5.8. I got this
error in the middle of the compilation process:

llvm[4]: Compiling InitHeaderSearch.cpp for Release+Asserts build
/Users/georgebaah/llvm3.1/llvm-3.1.src/tools/clang/lib/Frontend/InitHeaderSearch.cpp:29:10: fatal error:
‘clang/Config/config.h’ file not found
#include “clang/Config/config.h” // C_INCLUDE_DIRS
/Users/georgebaah/llvm3.1/llvm-3.1.src/tools/clang/lib/Frontend/InitHeaderSearch.cpp:237:26: error: use of undeclared
identifier ‘C_INCLUDE_DIRS’
StringRef CIncludeDirs(C_INCLUDE_DIRS);
2 errors generated.
make[4]: *** [/Users/georgebaah/llvm3.1/llvm-build/tools/clang/lib/Frontend/Release+Asserts/InitHeaderSearch.o] Error 1
make[3]: *** [Frontend/.makeall] Error 2

Can someone let me know what the problem is? Thanks.