Build Clang.jl using an external LLVM/Clang

I build julia with `USE_SYSTEM_LLVM=1` in `Make.user`, so there's no files about llvm/clang under my `JULIA_ROOT`, this fails the Clang.jl building.

So I change the way to get building flags for libclang: using llvm-config, and we can specify a llvm-config by set an environment variable `LLVM_CONFIG`, if no llvm-config is specified, the one under `JULIA_ROOT/usr/bin` will be used.

Since the external LLVM/Clang has no CXCursor.h,  USE_CLANG_CPP only works with the LLVM/Clang built with julia.

I also did few code format, hope that's OK.