fix load_all()

I think there is a mistake line 37 of compile-dll.r: "inst" must be inst.
Otherwise file.copy quietly fails.
I also suggest to look at the returned boolean and throw an error.
Problem when .onLoad package function is e.g.:
.onLoad <- function(lib, pkg)
{
 library.dynam("mypkg", pkg, lib)
}
it looks for "mypkg.so" in "<lib>/<pkg>/libs" directory and fails
because a previous call to compile_dll has copied mypkg.so in "<lib>/<pkg>/src" directory