rustdoc: Tell rustc we're building a library

This makes sure that when generating documentation we don't waste time doing
things like looking for a main function.

src/librustdoc/core.rs

@@ -51,6 +51,7 @@ fn get_ast_and_resolve(cpath: &Path,
         binary: @"rustdoc",
         maybe_sysroot: Some(@os::self_exe_path().unwrap().dir_path()),
         addl_lib_search_paths: @mut libs,
+        outputs: ~[driver::session::OutputDylib],
         .. (*rustc::driver::session::basic_options()).clone()
     };