Initial configure support for using Clang as the LLVM capable compiler.

Comes in two parts:
 1. Use --with-clang=path/to/clang/compiler to select an installed clang, or
    --with-built-clang to have the makefiles use the clang which will be built
    as the LLVM capable compiler. If neither is given, --with-built-clang will
    be used if the Clang sources are checked out into the standard location
    (tools/clang).

 2. Use --with-llvmcc={llvm-gcc,clang,none} to specify which LLVM capable
    compiler to use. If not given, then llvm-gcc will be used if available,
    otherwise Clang.

Makefile support still to come.

Eric, Doug, Chris, seem reasonable?

llvm-svn: 96934