Don't duplicate build files

Turns out keeping files in place, instead of symlinking or hard linking into
build/ means that Xcode can actually break on files in src/. This is cool!

There's still a problem with Xcode not getting appropriate type information
though.

SConstruct

@@ -149,7 +149,8 @@ def do_sconscript(env, build_env, src_dir, out_dir):
     # build_env is the Environment we're using to put everything together.
     env.SConscript(sconscript,
                    {'env': build_env, 'build_env': env},
-                   variant_dir=out_dir)
+                   variant_dir=out_dir,
+                   duplicate=0)
 
 
 debug_env = create_env('debug', {