Currently this is only enabled in the top-level as that's the only
place where it seemed to be used. But the libc/sys/phoenix/ dir
also uses this functionality, but fails to explicitly enable it.
Automake workedaround it, but generated warnings. Move the option
to NEWLIB_CONFIGURE so all dirs get it automatically iff they end
up using the option. If they don't use the option, there's no
difference to the generated code.
a492b26065