Remove usages of old style cast, use static_cast instead

This silences compiler warnings generated by GCC

include/boost/mp11/algorithm.hpp

@@ -309,7 +309,7 @@ template<template<class...> class L, class... T, template<class...> class L2, cl
 {
     template<class... W> static mp_identity<L<W...>> f( U*..., mp_identity<W>*... );
 
-    using R = decltype( f( (mp_identity<T>*)0 ... ) );
+    using R = decltype( f( static_cast<mp_identity<T>*>(0) ... ) );
 
     using type = typename R::type;
 };