The __llvm__ and __clang__ macros are the official way to check for an LLVM compiler (llvm-gcc or clang) or clang, respectively.
__has_feature and __has_builtin are the recommended way of checking for optional compiler features when using clang, they are documented here.
Note that you can find a list of the builtin compiler macros for gcc, llvm-gcc, and clang using:
echo | clang -dM -E -
This preprocesses an empty string and spits out all macros defined by the compiler.