Add option to use external llvm/clang and boost

[likebreath]

llvm/clang and boost now are all integrated as a part of the compilation of crete. This is good in terms of automation for building process.

A related inconvenience imposed by this is deleting a build tree of crete needs to rebuild llvm/clang and boost, which is very time consuming. A potential enhancement for this is to add an option to use external llvm/clang and boost.

A useful reference can be found here.